TESTS: add test for unreachable container

resulting in an empty `upstream {}` block in the generated nginx config file

test/stress_tests/test_unreachable_network/README.md

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+# nginx-proxy template is not considered when a container is not reachable
+
+Having a container with the `VIRTUAL_HOST` environment variable set but on a network not reachable from the nginx-proxy container will result in nginx-proxy serving the default nginx welcome page for all requests.
+
+Furthermore, if the nginx-proxy in such state is restarted, the nginx process will crash and the container stops.
+
+In the generated nginx config file, we can notice the presence of an empty `upstream {}` block.
+
+This can be fixed by merging [PR-585](https://github.com/jwilder/nginx-proxy/pull/585).
+
+## How to reproduce
+
+1. a first web container is created on network `netA`
+1. a second web container is created on network `netB`
+1. nginx-proxy is created with access to `netA` only
+
+
+## Erratic behavior
+
+- nginx serves the default welcome page for all requests to `/` and error 404 for any other path
+- nginx-container crash on restart
+
+Log shows:
+
+```
+webB_1          | starting a web server listening on port 82
+webA_1          | starting a web server listening on port 81
+reverseproxy    | forego     | starting dockergen.1 on port 5000
+reverseproxy    | forego     | starting nginx.1 on port 5100
+reverseproxy    | dockergen.1 | 2017/02/20 01:10:24 Generated '/etc/nginx/conf.d/default.conf' from 3 containers
+reverseproxy    | dockergen.1 | 2017/02/20 01:10:24 Running 'nginx -s reload'
+reverseproxy    | dockergen.1 | 2017/02/20 01:10:24 Error running notify command: nginx -s reload, exit status 1
+reverseproxy    | dockergen.1 | 2017/02/20 01:10:24 Watching docker events
+reverseproxy    | dockergen.1 | 2017/02/20 01:10:24 Contents of /etc/nginx/conf.d/default.conf did not change. Skipping notification 'nginx -s reload'
+reverseproxy    | reverseproxy    | forego     | starting dockergen.1 on port 5000  <---- nginx-proxy container restarted
+reverseproxy    | forego     | starting nginx.1 on port 5100
+reverseproxy    | dockergen.1 | 2017/02/20 01:10:24 Generated '/etc/nginx/conf.d/default.conf' from 3 containers
+reverseproxy    | dockergen.1 | 2017/02/20 01:10:24 Running 'nginx -s reload'
+reverseproxy    | dockergen.1 | 2017/02/20 01:10:24 Error running notify command: nginx -s reload, exit status 1
+reverseproxy    | dockergen.1 | 2017/02/20 01:10:24 Watching docker events
+reverseproxy    | dockergen.1 | 2017/02/20 01:10:24 Contents of /etc/nginx/conf.d/default.conf did not change. Skipping notification 'nginx -s reload'
+reverseproxy    | forego     | starting dockergen.1 on port 5000
+reverseproxy    | forego     | starting nginx.1 on port 5100
+reverseproxy    | nginx.1    | 2017/02/20 01:11:02 [emerg] 17#17: no servers are inside upstream in /etc/nginx/conf.d/default.conf:64
+reverseproxy    | forego     | starting nginx.1 on port 5200
+reverseproxy    | forego     | sending SIGTERM to nginx.1
+reverseproxy    | forego     | sending SIGTERM to dockergen.1
+reverseproxy exited with code 0
+reverseproxy exited with code 0
+
+```
+
+## Expected behavior
+
+- no default nginx welcome page should be served
+- nginx is able to forward requests to containers of `netA`
+- nginx respond with error 503 for unknown virtual hosts
+- nginx is not able to forward requests to containers of `netB` and responds with an error
+- nginx should survive restarts

test/stress_tests/test_unreachable_network/docker-compose.yml

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+version: "2"
+
+networks:
+  netA:
+  netB:
+
+services:
+  reverseproxy:
+    container_name: reverseproxy
+    networks:
+      - netA
+    image: jwilder/nginx-proxy:test
+    volumes:
+      - /var/run/docker.sock:/tmp/docker.sock:ro
+
+  webA:
+    networks:
+      - netA
+    image: web
+    expose:
+      - 81
+    environment:
+      WEB_PORTS: 81
+      VIRTUAL_HOST: webA.nginx-proxy
+
+  webB:
+    networks:
+      - netB
+    image: web
+    expose:
+      - 82
+    environment:
+      WEB_PORTS: 82
+      VIRTUAL_HOST: webB.nginx-proxy
+

test/stress_tests/test_unreachable_network/test_unreachable_net.py

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+from time import sleep
+
+import pytest
+import requests
+
+pytestmark = pytest.mark.xfail()  # TODO delete this marker once #585 is merged
+
+
+def test_default_nginx_welcome_page_should_not_be_served(docker_compose, nginxproxy):
+    r = nginxproxy.get("http://whatever.nginx-proxy/", allow_redirects=False)
+    assert "<title>Welcome to nginx!</title>" not in r.text
+
+
+def test_unknown_virtual_host_is_503(docker_compose, nginxproxy):
+    r = nginxproxy.get("http://unknown.nginx-proxy/", allow_redirects=False)
+    assert r.status_code == 503
+
+
+def test_http_web_a_is_forwarded(docker_compose, nginxproxy):
+    r = nginxproxy.get("http://webA.nginx-proxy/port", allow_redirects=False)
+    assert r.status_code == 200
+    assert "answer from port 81\n" == r.text
+
+
+def test_http_web_b_gets_an_error(docker_compose, nginxproxy):
+    r = nginxproxy.get("http://webB.nginx-proxy/", allow_redirects=False)
+    assert "<title>Welcome to nginx!</title>" not in r.text
+    with pytest.raises(requests.exceptions.HTTPError):
+        r.raise_for_status()
+
+
+def test_reverseproxy_survive_restart(docker_compose):
+    docker_compose.containers.get("reverseproxy").restart()
+    sleep(2)  # give time for the container to initialize
+    assert docker_compose.containers.get("reverseproxy").status == "running"