Add skip check for volume revert feature

Volume revert is a new feature added in microversion 3.40, and now not all
storage backend drivers have supported this feature. So it is necessary to
add a skip check for it in Tempest tests.

Change-Id: Icbd108f62f5b308e419843b03af20c8fe1278b24
diff --git a/cinder/tests/tempest/config.py b/cinder/tests/tempest/config.py
index f4eb098..e15a399 100644
--- a/cinder/tests/tempest/config.py
+++ b/cinder/tests/tempest/config.py
@@ -19,4 +19,7 @@
     cfg.BoolOpt('consistency_group',
                 default=False,
                 help='Enable to run Cinder volume consistency group tests'),
+    cfg.BoolOpt('volume_revert',
+                default=False,
+                help='Enable to run Cinder volume revert tests'),
 ]