Git fixes for building testing repos

We should always clone from git.openstack.org, the repo in
zuul is only used to merge refs to zuul's internal purposes.

Revert "Always build a new git repo" (commit
bae030499545d40f450a543eaa906dc6c55e88d1) as this will be too
expensive for git.openstack.org.

Change-Id: Id2e3f92fbdba512b5b8c6f9bd27cd880dbb21f9f