Nova snapshot fail with multiple rbd stores

As of now, with multi store enabled, adding a new location to an image will make the add_location API call to fail if the store metadata is missing:

Code in glance: https://github.com/openstack/glance/blob/master/glance/location.py#L134
Then in glance_store: https://github.com/openstack/glance_store/blob/master/glance_store/location.py#L111

This will raise a "KeyError: None" and raise a very standard "Invalid Location" 400 error when adding a new location.

Point is, with a rbd backend, nova never specify this metadata when creating the image during the direct snapshot process (flatten the image directly in ceph image pool + adding the location directly in glance), so snapshot will always fail.

A solution can be to infer the backend from the location uri, like we do during the store metadata lazy population.