Add git fetch --tags to gerrit-git-prep.sh.
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 <37c07a49ee3d5191376d758eb1d8298b34456b8e@inaugust.com>
Approved: James E. Blair <e81a02a9d6405977f63a769d9f6ec8ed9d924325@inaugust.com>
Reviewed-by: James E. Blair <e81a02a9d6405977f63a769d9f6ec8ed9d924325@inaugust.com>
Tested-by: Jenkins

open-switch/infra_puppet-jenkins,open-switch/infra_puppet-jenkins