Nodepool: cache all gerrit repos

Cache all review.openstack.org git repos in /opt/git.

Subsequent changes to gerrit-git-prep and devstack-gate will make
use of these repos (eventually deprecating the devstack-gate repos
is ~jenkins/workspace-cache).

Refactor the devstack cache python script a bit to make common
functions reusable.  Also rename it because underscores are better
than hyphens for python file names.

Change-Id: I4ae40613ebcca62b0dfc2eaba159557a4c71a529
Merge "Nodepool: cache all gerrit repos"