Updates for stable branch creation

Set default branch for git review/gerrit.

Switch amulet tests to stable.

Switch zaza bundles to stable charms.

Switch to using stable charm-helpers branch.

Change-Id: Ice84be31ced1908aded21d9f58296bd581bbcd91

.gitreview

@@ -2,3 +2,5 @@
 host=review.opendev.org
 port=29418
 project=openstack/charm-cinder-ceph.git
+
+defaultbranch=stable/20.02

charm-helpers-hooks.yaml

@@ -1,4 +1,4 @@
-repo: https://github.com/juju/charm-helpers
+repo: https://github.com/juju/charm-helpers@stable/20.02
 destination: charmhelpers
 include:
     - core

tests/basic_deployment.py

@@ -38,7 +38,7 @@ class CinderCephBasicDeployment(OpenStackAmuletDeployment):
     """Amulet tests on a basic heat deployment."""
 
     def __init__(self, series=None, openstack=None, source=None, git=False,
-                 stable=False):
+                 stable=True):
         """Deploy the entire test environment."""
         super(CinderCephBasicDeployment, self).__init__(series, openstack,
                                                         source, stable)