Merge branch 'stable-3.1'

* stable-3.1:
  Set Gerrit to 3.1.6
  Set Gerrit to 3.0.10
  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
  Set Gerrit to 3.1.5
  Consume uploadvalidator from its own stable-3.1 branch
  Set Gerrit to 3.0.9
  Set version to 2.16.19
  Set version to 2.16.18

Change-Id: I4438d78cb0377ff5694a99e72684defde3fcc402