Check that when multiple containers have the same WIRTUAL_HOST and one of
them is unreachable, the resulting `upstream` block has no
`server 127.0.0.1 down;` entry.
The VIRTUAL_PORT environment variable should always be honored.
Even when the related port is not exposed.
Fix for nging-proxy/nginx-proxy#1132.
This commit also add the DEBUG environment variable which enables more
verbose comments in the nginx comfiguration file to help troubleshooting
unreachable containers.
Finaly it fixesnging-proxy/nginx-proxy#1105 as well by defining only one
fallback entry per upstream block.