Updates for stable branch creation

Set default branch for git review/gerrit.

Switch amulet tests to stable.

Switch to using stable charm-helpers branch.

Change-Id: I99cd836f26f7b5acb4c4d4752d6ad891e78081eb

.gitreview

@@ -2,3 +2,5 @@
 host=review.openstack.org
 port=29418
 project=openstack/charm-rabbitmq-server.git
+
+defaultbranch=stable/18.11

charm-helpers-hooks.yaml

@@ -1,5 +1,5 @@
 destination: charmhelpers
-repo: https://github.com/juju/charm-helpers
+repo: https://github.com/juju/charm-helpers@stable/18.11
 include:
     - fetch
     - core

tests/basic_deployment.py

@@ -55,7 +55,7 @@ class RmqBasicDeployment(OpenStackAmuletDeployment):
     """Amulet tests on a basic rabbitmq cluster deployment. Verify
        relations, service status, users and endpoint service catalog."""
 
-    def __init__(self, series=None, openstack=None, source=None, stable=False):
+    def __init__(self, series=None, openstack=None, source=None, stable=True):
         """Deploy the entire test environment."""
         super(RmqBasicDeployment, self).__init__(series, openstack, source,
                                                  stable)