Merge branch 'stable-3.0' into stable-3.1

* stable-3.0:
  Remove plugin-manager from the list of core plugins
  Remove BUCK build
  Remove check with hardcoded GERRIT_NEXT_VERSION
  Fix core plugin name extractions without MANIFEST.MF
  Backport tests from master to stable-2.16
  Set next version to 3.1

Change-Id: I4c4cab0cec3fc9db56e2e5d01482d826e642ee80