test/docker: Use gerritcodereview docker image

Leverage more work already done for us by layering on top of the
available 2.16 image from [1].

Retain the options for specifying a gerrit.war and/or task.jar but
convert them to use local paths since the use cases for those now are to
specify locally-built artifacts.

[1] https://hub.docker.com/r/gerritcodereview/gerrit

Change-Id: I1dfc87eac0301cf3a420dbd22104de4f91646eab
2 files changed