Run heat_integrationtests in parallel

This should improve the turnaround time of the gate tests and
also make room for more integration tests.

Also, remove duplicate addCleanup.

Change-Id: Iacdd554ee13bb87b0b71ebf547209f94e945f342
diff --git a/common/test.py b/common/test.py
index 7d3ea4e..7c8300d 100644
--- a/common/test.py
+++ b/common/test.py
@@ -363,7 +363,6 @@
         kwargs = {'stack_identifier': stack_identifier,
                   'status': expected_status}
         if expected_status in ['ROLLBACK_COMPLETE']:
-            self.addCleanup(self.client.stacks.delete, stack_name)
             # To trigger rollback you would intentionally fail the stack
             # Hence check for rollback failures
             kwargs['failure_pattern'] = '^ROLLBACK_FAILED$'