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: I77f0d890e6dd17d40de8131666eb97a29902ea47

.gitreview

@@ -2,3 +2,5 @@
 host=review.openstack.org
 port=29418
 project=openstack/charm-cinder-backup.git
+
+defaultbranch=stable/19.04

charm-helpers-hooks.yaml

@@ -1,4 +1,4 @@
-repo: https://github.com/juju/charm-helpers
+repo: https://github.com/juju/charm-helpers@stable/19.04
 destination: hooks/charmhelpers
 include:
     - core

tests/basic_deployment.py

@@ -36,7 +36,7 @@ class CinderBackupBasicDeployment(OpenStackAmuletDeployment):
     """Amulet tests on a basic cinder-backup deployment."""
 
     def __init__(self, series=None, openstack=None, source=None, git=False,
-                 stable=False):
+                 stable=True):
         """Deploy the entire test environment."""
         super(CinderBackupBasicDeployment, self).__init__(series, openstack,
                                                           source, stable)