mkcloud: Use upgrade_cloudsource to determine the upgrade version

The guesswork on where to upgrade to was faulty. Instead use
a upgrade_cloudsource that determines the version to upgrade to.
Then we can simply set the new cloudsource and let
h_set_source_variables do its magic.
qa_openstack.sh: Wait for cinder-volume to be ready

it seems on openSUSE 13.x and SLE12 cinder-volume needs a very
long time to start, so the cinder tests fail early on.