Automatically diagnosing known flakes

It often takes a long time to fix a flake; meanwhile it takes a lot of
time for sheriffs to identify whether to register a new flake for a
failure.

This CL adds automatic identifies identification of one know flake type.
Once identified, it rewrites the error so that:
1. Flakes clustering tool that has only rudimentary clustering, places
all flakes of this kind in the same cluster (not multiple ones like
now). This is a step towards using clustering tool for monitoring
flakes;
2. Sheriff immediately sees that the issue is known.

Change-Id: I86a0762665cb21434289e1be00b60bd76fec4142