Ceph Multibackend changes

The plain Encryptor provider is not support on ceph backends, the
test test_encrypted_cinder_volumes_cryptsetup fails on:
cinder.exception.VolumeDriverException: Volume driver reported an error:
Provider plain not supported.
This comes from [1] which is called in [2]. The Plain provider will be deprecated
in future completely.

[1] https://opendev.org/openstack/cinder/src/branch/master/cinder/volume/volume_utils.py#L1195
[2] https://opendev.org/openstack/cinder/src/branch/master/cinder/volume/drivers/rbd.py#L877

Related-Prod: PRODX-3456

Change-Id: Ib6290ed1ad5779e4e6fed449f3335dbdada42f0d
diff --git a/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py b/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py
index da7d408..873731f 100644
--- a/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py
+++ b/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py
@@ -12,6 +12,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import testtools
 import time
 
 from oslo_log import log as logging
@@ -124,6 +125,8 @@
                                               volume_type='luks')
         self.attach_detach_volume(server, volume, keypair)
 
+    @testtools.skipIf(CONF.volume.storage_protocol == 'ceph',
+                      'PLAIN encryptor provider is not supported on rbd')
     @decorators.idempotent_id('cbc752ed-b716-4727-910f-956ccf965723')
     @utils.services('compute', 'volume', 'image')
     def test_encrypted_cinder_volumes_cryptsetup(self):