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: I9946b3d45a3b4d83a0abffafa8b7af3943b76962

.gitreview

@@ -2,3 +2,5 @@
 host=review.openstack.org
 port=29418
 project=openstack/charm-nova-cloud-controller.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: charmhelpers
 include:
     - core

tests/basic_deployment.py

@@ -56,7 +56,7 @@ class NovaCCBasicDeployment(OpenStackAmuletDeployment):
     """Amulet tests on a basic nova cloud controller deployment."""
 
     def __init__(self, series=None, openstack=None, source=None,
-                 stable=False):
+                 stable=True):
         """Deploy the entire test environment."""
         super(NovaCCBasicDeployment, self).__init__(series, openstack,
                                                     source, stable)