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: I4b5e68b90310ef7206ceda5453c2fd7ecbb29bee

.gitreview

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

charm-helpers-hooks.yaml

@@ -1,4 +1,4 @@
-repo: https://github.com/juju/charm-helpers
+repo: https://github.com/juju/charm-helpers@stable/18.11
 destination: hooks/charmhelpers
 include:
     - core

tests/basic_deployment.py

@@ -35,7 +35,7 @@ class CeilometerBasicDeployment(OpenStackAmuletDeployment):
     no_origin = ['memcached', 'percona-cluster', 'rabbitmq-server',
                  'ceph-mon', 'ceph-osd']
 
-    def __init__(self, series, openstack=None, source=None, stable=False):
+    def __init__(self, series, openstack=None, source=None, stable=True):
         """Deploy the entire test environment."""
         super(CeilometerBasicDeployment, self).__init__(series, openstack,
                                                         source, stable)