dt's changes

* Move lib/storage_plugins/ceph -> lib/ceph
* Move lib/cinder_plugins/ceph -> lib/cinder_backends/ceph
* Refactor Cinder config to use cinder-backends
* Remove unnecessary changes to other DevStack files

Change-Id: I9c75a74f1bf08ee77487f22ad611a36cffcc2551
Merge branch 'cinder-backends' into dt-test
Merge branch 'add-pre-install' into dt-test
Merge branch 'new-ceph-backend' into dt-test
Merge branch 'test-ini-improvements' into dt-test
Merge branch 'usb-prereq' into dt-test
Merge branch 'conf-tempest' into dt-test