git scm module in jenkins should work better with gerrit

The git scm module should be made to work better with testing random gerrit branches that may not get merged or may conflict, etc. It should do the equivalent of this job:

https://jenkins.openstack.org/job/template-gerrit-git-prep/configure

Which at the time of writing is:

#!/bin/bash -x
if [[ ! -e .git ]]; then
    git clone https://review.openstack.org/p/$GERRIT_PROJECT .
fi
git remote update
git reset --hard
git clean -x -f -d -q
git checkout $GERRIT_BRANCH
git reset --hard remotes/origin/$GERRIT_BRANCH
git clean -x -f -d -q
git fetch https://review.openstack.org/p/$GERRIT_PROJECT $GERRIT_REFSPEC
git merge FETCH_HEAD