Skip test_encrypted_cinder_volumes_cryptsetup on Ceph

Ceph/RBD backend in cinder does not support plain encryption provider.

Related-Issue: PRODX-37481
Change-Id: Iaf5de3958ecbcd5696ab31708005440ccd3ab2de
(cherry picked from commit e08a6c414aaf4fa164efe68d26887b95611e0fc7)
diff --git a/tempest/scenario/test_encrypted_cinder_volumes.py b/tempest/scenario/test_encrypted_cinder_volumes.py
index 753e64f..95087cb 100644
--- a/tempest/scenario/test_encrypted_cinder_volumes.py
+++ b/tempest/scenario/test_encrypted_cinder_volumes.py
@@ -89,6 +89,8 @@
         self.attach_detach_volume(server, volume)
 
     @decorators.idempotent_id('cbc752ed-b716-4717-910f-956cce965722')
+    @testtools.skipIf(CONF.volume.storage_protocol == 'ceph',
+                      'Skip because ceph does not support Provider plain.')
     @decorators.attr(type='slow')
     @utils.services('compute', 'volume', 'image')
     def test_encrypted_cinder_volumes_cryptsetup(self):