Allow developer to disable all future builds on WIP PRs

[blomquisg]

When working on a feature, sometimes a developer (me) will push multiple changes up to github as either rebases or stream of consciousness commits. In this case, the developer should be able to disable the Travis builds until some later time when the developer thinks the PR is ready to move forward.

It may not make a huge impact, but it would likely cut down on the number of extra uneccessary builds we're asking Travis to handle.

[Fryguy]

Maybe the bot could add a label called "builds disabled", so the user knows, and could adjust with @miq-bot cancel builds @miq-bot enable builds

[miq-bot]

This issue has been automatically marked as stale because it has not been updated for at least 3 months.

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