Have buildot build changes submitted for review on Gerrit

[pulkomandy]

It would be nice to have uildbot check that changes submitted on Gerrit do not break the build. Later on we can also integrate running haiku-format or other checks, too.

Changes needed:

• The buildbot is currently triggered from a hook on Gerrit side. The hook could be modified to let refs/for/ changes go to buildbot, but I'm not sure if that would give buildbot enough info to submit its results
• Buildbot recent versions have a GerritChangeSource (​http://docs.buildbot.net/latest/manual/configuration/changesources.html#chsrc-GerritChangeSource), which sounds worth investigating (the current buildbot config has "Y U NO WORK" as a comment for the Git change source which is similar, but I don't know what the problem is with it from this very short description, and there is no ticket about it).
• Buildbot also has a Gerrit buildstep (​http://docs.buildbot.net/latest/manual/configuration/buildsteps.html?highlight=gerrit#gerrit) which integrates with the above to build separate changes in the same project, without creating a dedicated build for each. It can also be used without the changesource if we provide it with the extra info (so if we don't get the changesource working, it's still possible to use this part at least)
• Gerrit has a REST API to submit the results. The bot can automatically +1 or -2 the build (in a category separate from human code reviews)
• Gerrit is planning an API for inline review comments, so it could even pinpoint the exact place where there is a build error. But I think this is not in current releases yet.