[capacity] Fix TasksCfg lookup and bot grouping
[capacity] Fix TasksCfg lookup and bot grouping

https://status.skia.org/capacity was not showing NVIDIA_Shield and I
couldn't figure out why based on logs due to the enormous number of
warnings for "TaskSpec X is not found in repo Y" when X actually belongs
to repo Z.

Since I was fixing the TaskSpec lookup to ensure we take repo into
account, I figured I would also handle the case when the TaskCfgCache
hasn't yet cached the latest TasksCfg.

After doing that, I wondered if the problem is actually in
mergeBotConfigs due to moving devices to different bots. Inspecting the
JSON revealed that was indeed the case. Added a fix to avoid grouping
bots whose dimensions conflict.

Change-Id: I7876cfbf0d32373c1abbd9874ce59108b685b55c
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/222780
Commit-Queue: Ben Wagner aka dogben <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@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