Release charts via GitHub label

[timothyb89]

It would be nice if we could trigger chart releases via labels.

Ideally we'd have three labels: release-major, release-minor, and release-patch. If one (and only one) of these is applied to a PR, the charts versions modified in that PR should be incremented.

This could happen in one of two places:

• push.sh could just immediately increment the version as part of the merge
• the pr bot could follow up with a patch to increment the version

The pr-bot is probably the better solution but would take more work to implement. Incrementing the version behind the scenes feels a little creepy but would be easier, especially since our CI script already writes to gh-pages.

For bonus points, a comment can be left on the PR to notify about the release.

It's pretty easy to get a list of labels via the GitHub API:

[tim:~]↥ % http get https://api.github.com/repos/monasca/monasca-helm/issues/237 | jq -r '.labels[].name'
enhancement