factor out gerrit project creator account

this lets you set the gerrit project creator account in yaml,
instead of it being hardcoded in the script

Change-Id: I08870255fd3f44b8e2ed04ed7a7e80a3dfe2dd39
Reviewed-on: https://review.openstack.org/28286
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins