Skip rebuild_volume_backed_server if no cinder

This is a compute test that requires cinder, but we do class-based
setup before we check the skip flag on the test. Do this in class-
based skip_checks() to prevent it from running.

Change-Id: I028f299381793113a1415a9972715ef94b409492
diff --git a/tempest/api/compute/servers/test_server_actions.py b/tempest/api/compute/servers/test_server_actions.py
index 3e54bf6..59cf360 100644
--- a/tempest/api/compute/servers/test_server_actions.py
+++ b/tempest/api/compute/servers/test_server_actions.py
@@ -831,6 +831,12 @@
     volume_min_microversion = '3.68'
 
     @classmethod
+    def skip_checks(cls):
+        if not CONF.service_available.cinder:
+            raise cls.skipException("Cinder is not available")
+        return super().skip_checks()
+
+    @classmethod
     def setup_credentials(cls):
         cls.prepare_instance_network()
         super(ServerActionsV293TestJSON, cls).setup_credentials()