Add OwnersValidator to check changed OWNERS.

* Checks OWNERS file syntax and email addresses used in OWNERS files.
  Emails should belong to accounts found on the Gerrit server.
* This check is disabled by default and enabled in project.config with
  "rejectErrorInOwners = true".

Change-Id: I2fe59bd622dd118d4c59bbf6a1fb574b3a58a896