Build gerrit plugins against 2.13.7 release
Build gerrit plugins against 2.13.7 release

The plan is to upgrade gerrit to 2.13.7 release so update jobs to
build Gerrit plugins against that release.

Change-Id: I3360447178e5f808e15f011db083460195809cde

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