Change SkaffoldOption Labeller to not include a comma in the label value

[GeertJohan]

When running skaffold to deploy to my cluster, I get tens of errors like this:

time="2018-10-16T10:03:34Z" level=warning msg="error adding label to runtime object: patching resource nlx-test-haarlem/txlog-ui: Ingress.extensions \"txlog-ui\" is invalid: metadata.labels: Invalid value: \"release,deploy-test\": a valid label must be an empty string or consist of alphanumeric characters, '-', '_' or '.', and must start and end with an alphanumeric character (e.g. 'MyValue',  or 'my_value',  or '12345', regex used for validation is '(([A-Za-z0-9][-A-Za-z0-9_.]*)?[A-Za-z0-9])?')"

Comma's are not allowed in label values. Skaffold uses them when multiple profiles are used.

This simple fix changes the separator from comma to a double underscore.

[dgageot]

@GeertJohan +1 for the fix! You have to fix the TestLabels test though

[codecov-io]

Codecov Report

Merging #1169 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1169   +/-   ##
=======================================
  Coverage   42.18%   42.18%           
=======================================
  Files          89       89           
  Lines        3999     3999           
=======================================
  Hits         1687     1687           
  Misses       2143     2143           
  Partials      169      169

Impacted Files	Coverage Δ
pkg/skaffold/config/options.go	88.23% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update aad90b0...3388987. Read the comment docs.

[r2d4]

Thank you 👍