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: Ie135301985786c6da283816acb69c51a8cdc15d3

.gitreview

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

tests/basic_deployment.py

@@ -34,7 +34,7 @@ u = OpenStackAmuletUtils(DEBUG)
 class SwiftStorageBasicDeployment(OpenStackAmuletDeployment):
     """Amulet tests on a basic swift-storage deployment."""
 
-    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(SwiftStorageBasicDeployment, self).__init__(series, openstack,
                                                           source, stable)