The "additional_plugins" variable is so different builds gerrit can
specify additional plugins specific to their version to install into
the base image.
Since we've moved to only building 3.2 and master images, a bunch of
plugins that used to be additional (because they weren't 2.XX era) are
now common. Move them into the common plugin code in the playbook,
and leave the only one different for master, the "checks" plugin, as
separate.
Change-Id: I8966ed7b5436fbe012486dccc1028bc8cb1cf9e4
1858f15a8a