specify ceph mirror

http://ci.openstack.org/jenkins-job-builder/properties.html#properties.github

is probably the wrong place to specify the mirror and

http://ci.openstack.org/jenkins-job-builder/scm.html#scm.git

looks like a better candidate.

Signed-off-by: Loic Dachary <loic@dachary.org>
LGTM