Use openstack/2.13 branch to build gerrit plugins
Use openstack/2.13 branch to build gerrit plugins

Rather than track each individual point release on a different branch
just create a 2.13 branch which can be updated for 2.13.9 and so on and
use that.

Change-Id: I7b74bbaed35bed62f7b7fc18b630f59d0d1c3903

openstack-infra/project-config,openstack-infra/project-config