Segments Labels: Mesos

[drewkerrigan]

What does this PR do?

• Add support for segment labels for Mesos
  • Also added behavior such that if the segment name matches a named port, and no other port selector labels are used, then that port will be used for that segment
• Add new traefik.portName label
  • Only used by Mesos currently, but could potentially be used by Marathon as well
  • Used as port selector for mesos tasks with multiple named ports
• Added tests in provider/mesos/config_test.go for both of the above
• Added segment label documentation to docs/configuration/backends/mesos.md
• Added some additional template functions (not required by existing templates, so backwards compatible)

Motivation

Segment labels were missing from the mesos provider and they are obviously very useful and sometimes required for mesos frameworks running many multi-port tasks.

Related to #3383

More

• Added/updated tests
• Added/updated documentation

Additional Notes

Regarding the design: My primary goal was to keep these changes backwards compatible with the existing templates, so I kept the taskData object basically the same except for the addition of the SegmentName. This approach means that when there are segments, the state.Task is duplicated per segment, but it also means that the majority of the code and templates using taskData.TraefikLabels and the embedded state.Task can remain the same.

[ldez]

LGTM

[dtomcej]

LGTM

[juliens]

LGTM