workaround for jenkins bug https://issues.jenkins-ci.org/browse/JENKINS-14193

The jenkins maven builder cannot reference environment variables if it is set
by the EnvInject plugin at the prebuilder step.  However the maven builder CAN
reference variables when loaded from the build environment setup (wrapper) step.
The only problem is that gerrit-git-prep runs in prebuilder step which occurs after
a build environment step.  We want the script to run after gerrit-git-prep so
we are calling the gerrit-git-prep in script so it will run in correct order.

Change-Id: Iedd7561b4ef8e19b3713b63bb76b9b754ce116e9
Reviewed-on: https://review.openstack.org/34023
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins