Make Gerrit system user and group configurable

The gerrit2 system user and group was hardcoded in
manage_projects. This simply allows the values to
be overridden by options 'gerrit-system-user' and
'gerrit-system-group' in the defaults section of the
projects.yml file.

fixes lp 1191542

Change-Id: Ieb736e08a6f96c10abfabe8b17808681732cb8b6
Reviewed-on: https://review.openstack.org/33196
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins