Support multiple Cinder backend types

This is the first step in supporting multiple Cinder backend types at
once.  It initially converts the existing hard-coded multi-lvm support
to a new cinder_backends driver form.  Eventually the cinder_plugins
will be converted to this form so they can be enabled more than just
one at a time.

The default configuration should be identical to the previous defaults,
including for both True and False values of CINDER_MULTI_LVM_BACKEND.

The existing cinder_plugins are expected to be removed when this is
complete. They should continue to work until they have been converted.

Change-Id: I93b8ef32832269d730c76a6dc24ddb4f20c6d9df