If no volume, don't delete volume in teardown

If exception is thrown in setupClass before volume is created,
tearDownClass needn't delete volume.

Change-Id: I91c7eff8ec147b0ce3a9ea3e6c8d906ba68412c2
Closes-Bug: 1350167
diff --git a/tempest/api/compute/v3/servers/test_server_rescue_negative.py b/tempest/api/compute/v3/servers/test_server_rescue_negative.py
index 5eb6c9a..022c963 100644
--- a/tempest/api/compute/v3/servers/test_server_rescue_negative.py
+++ b/tempest/api/compute/v3/servers/test_server_rescue_negative.py
@@ -56,7 +56,8 @@
 
     @classmethod
     def tearDownClass(cls):
-        cls.delete_volume(cls.volume['id'])
+        if hasattr(cls, 'volume'):
+            cls.delete_volume(cls.volume['id'])
         super(ServerRescueNegativeV3Test, cls).tearDownClass()
 
     def _detach(self, server_id, volume_id):