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: Iec07bd6c658a07ca914dafe1f04238512606a07d

.gitreview

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

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

@@ -45,7 +45,7 @@ class LXDBasicDeployment(OpenStackAmuletDeployment):
     """Amulet tests on a basic nova compute deployment."""
 
     def __init__(self, series=None, openstack=None, source=None,
-                 stable=False):
+                 stable=True):
         """Deploy the test environment."""
         super(LXDBasicDeployment, self).__init__(series, openstack,
                                                  source, stable)