Backup backend switch

It is a viable scenario that openstack operator may want to change backup
backend.

There should be a way to switch to a different backup backend on existing
cinder deployment, where data is already stored without losing the data.

The idea is to transition from old to new backup backend as transparently as
possible from end-user perspective, without the need to move data between
backends, or any action by end-user.

Once the switch is done, no new data should be written to old backend.

Scope of this blueprint/spec may be limited to backup drivers based on
ChunkedDriver.