Instead of relying on git_base_url, use the get_ceph_git_url() to obtain
the URL of the Ceph repository to use with git clone. This allows the
user to override it via the git_ceph_url configuration option and the
--git-ceph-url command line option to teuthology-openstack.
http://tracker.ceph.com/issues/11883 Refs: #11883
Signed-off-by: Loic Dachary <loic@dachary.org>