Check //testing/buildbot configs for milo changes

//testing/buildbot scripts look at luci-milo.cfg to see what bots exist,
to make sure people don't define bots which don't exist. If a CL changes
luci-milo.cfg, this can break those scripts. Currently this isn't
detected at all, since the presubmit scripts which would detect this
live in //testing/buildbot, and aren't run, if the CL only touches
//infra/config/global/luci-milo.cfg. This CL checks to see if a CL
touches milo configs. If it does, it runs the //testing/buildbot config
validation logic.

Change-Id: I7b51ea7442b5ab361b9d4c3b1c696043f22ed52a
Reviewed-on: https://chromium-review.googlesource.com/c/1383214
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617705}
3 files changed