Ensure core plugins are bundled by Maven

Require the core plugins whenever a Gerrit build is run.
These are pulled into the Maven Reactor by being inside
of git submodules. Plugins can be omitted by passing a
-Dgerrit.plugins.skip=true flag on the command line.

Change-Id: I7246389bcae9e98d5f1d46208ff515855eb7c474
11 files changed