Switch to using git.openstack.org

- All openstack repositories are now hosted on git.openstack.org
- Github.com hasn't been reliable for our vm-gate tests

Change-Id: I6957e2dcbfb9b065f36775e2adfa045604388122
Merge "Switch to using git.openstack.org"
Updating diskimage-builders repo

Change-Id: I07ac7f97605d2413d5d27a1a3d66f7bdbfc1e053