The IP-Per-Task feature changed the behavior for
clients without this configuration (using the task IP instead
of task hostname). This patch make the new behavior available
just for Mesos installation with IP-Per-Task enabled. It also
make it possible to force the use of task's hostname.
Previously, we did the check too late resulting in the traefik.port
label not being effective.
The change comes with additional refactorings in production and tests.
