Add missing gerrit-git-prep on run-tests

I forgot to add these in my run-tests template, so trying to run
./run-test.sh without cloning first the project does not work at all.

Change-Id: I18abba2cea5b924204af2125f6c49c6a86cf0106
Reviewed-on: https://review.openstack.org/35125
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Elizabeth Krumbach Joseph <lyz@princessleia.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins