Merge branch 'stable-3.3'

* stable-3.3:
  Align testcontainers with v1.15 in Gerrit
  Add zookeeper-refdb jar compiled with zookeeper 3.5.x
  Add SSL support for Zookeeper client
  Jenkinsfile: use gerrit-ci-library pipeline for plugin validation
  Revert "Add SSL support for Zookeeper client"
  Add SSL support for Zookeeper client

Change-Id: I7736637c9467e8ad3ba3b975abc8dae73aec07bf