Check for explicitly defined Marathon port first.

[timoreimann]

Check for explicitly defined Marathon port first.

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.

Original issue at #261.
Complements PR at #1394.
Discovered along #1390.

[timoreimann]

The initial push contains an updated test only to demonstrate the problem. Once CI goes legitimately red, the fix will be pushed.

[timoreimann]

@containous/traefik we should get this into 1.3 as the traefik.port label override is currently broken with Marathon.

Sorry for the big delta in the tests, but the fact that they did not adequately address/test-cover the code is one of the reasons why the problem didn't surface earlier. Thus, I needed to shift things around.

[vdemeester]

LGTM 👼

1. We really need to enhance these tests (mainly removing noise just like we did on docker provider)
2. @ttr you probably want to remove the first commit before we merge (otherwise it is going to break bisect if we merge it without squashing 😛 — I think 🤔 )

[timoreimann]

@vdemeester

1. I plan to follow your good example and do some cleanup on Marathon. I need to add support for readiness checks in the foreseeable future, which will require doing requests against the Marathon API slightly different (it'll be one combined request per event update instead of two (apps + tasks)). This will require overhauling the tests anyway, so I'm going to take that opportunity.
2. You're definitely right, thanks for the reminder. Will do so right away.

@ttr: Sorry for the noise (again) -- I should sync my Slack and Github handles in the future. :-)