Support multiple Cinder backends

This is the first step in supporting multiple Cinder backends 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
Merge branch 'usb-prereq' into dt-test
Merge branch 'conf-tempest' into dt-test