Add git fetch --tags to gerrit-git-prep.sh.

* modules/jenkins/files/slave_scripts/gerrit-git-prep.sh: Run
`git fetch --tags origin` to make sure that we have all available
tags`.

Change-Id: I2cfd691659cd0e10f5295fbbe66b36b818d6d6b6
Reviewed-on: https://review.openstack.org/35527
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins