Build gerrit plugins against 2.13.8
Build gerrit plugins against 2.13.8

We are going to do gerrit 2.13.8 after trouble with 2.13.7 and the weird
DB migration required for an upgrade from 2.13.7 to 2.13.8. This
requires building new plugins against this version of gerrit.

Change-Id: Ic7dbbe090f0f8c7b64d3671798c207f7eb548eb9

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