fix: Don't create fallback http(s) server when http(s) disabled

Before, a fallback http server was created to handle requests for
unknown virtual hosts even when `HTTPS_METHOD=nohttp`.  (In this case,
all http vhosts would be unknown.)  Likewise, a catch-all fallback
https server was still created even if `HTTPS_METHOD=nohttps`.

Now the fallback servers are created only if needed.  This brings the
behavior in line with the documentation and user expectation.  It will
also make it easier to implement a planned feature: different servers
on different ports.

test/test_fallback.data/nohttps.yml

@@ -0,0 +1,14 @@
+services:
+  sut:
+    image: nginxproxy/nginx-proxy:test
+    volumes:
+      - /var/run/docker.sock:/tmp/docker.sock:ro
+    environment:
+      HTTPS_METHOD: nohttps
+  http-only:
+    image: web
+    expose:
+      - "83"
+    environment:
+      WEB_PORTS: "83"
+      VIRTUAL_HOST: http-only.nginx-proxy.test