Exclude CT GCE bots from BotMissing alerts and add a new alert for them
Exclude CT GCE bots from BotMissing alerts and add a new alert for them

Bug: skia:6648
Change-Id: I1f318825c57dc1067265764a8b9b2c01eb32cdd9
Reviewed-on: https://skia-review.googlesource.com/29122
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot