Merge branch 'stable-2.16' into stable-3.0

* stable-2.16:
  Set Gerrit to 2.16.21
  Set Gerrit to 2.15.19
  Fetch plugins from archive-ci.gerritforge.com
  Set Gerrit to 2.14.21
  Set version to 2.16.20

Change-Id: I30cd65fd6fbf3fea5cbffebdf51b0ac1866dd14f