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: Ie5e48ef63411be9c32273246389fccffc66e1f9f

.gitreview

@@ -2,3 +2,5 @@
 host=review.openstack.org
 port=29418
 project=openstack/charm-ceph.git
+
+defaultbranch=stable/17.11

charm-helpers-hooks.yaml

@@ -1,4 +1,4 @@
-repo: https://github.com/juju/charm-helpers
+repo: https://github.com/juju/charm-helpers@stable/17.11
 destination: hooks/charmhelpers
 include:
     - core

charm-helpers-tests.yaml

@@ -1,4 +1,4 @@
-repo: https://github.com/juju/charm-helpers
+repo: https://github.com/juju/charm-helpers@stable/17.11
 destination: tests/charmhelpers
 include:
     - contrib.amulet

tests/basic_deployment.py

@@ -33,7 +33,7 @@ u = OpenStackAmuletUtils(DEBUG)
 class CephBasicDeployment(OpenStackAmuletDeployment):
     """Amulet tests on a basic ceph deployment."""
 
-    def __init__(self, series=None, openstack=None, source=None, stable=False):
+    def __init__(self, series=None, openstack=None, source=None, stable=True):
         """Deploy the entire test environment."""
         super(CephBasicDeployment, self).__init__(series, openstack, source,
                                                   stable)