Updates for 18.08 stable branch creation

Set default branch for git review/gerrit.

Switch amulet tests to stable.

Switch to using stable charm-helpers branch.

Change-Id: I1714094778e991fca3a0d90a3f903e10ae454905

.gitreview

@@ -2,3 +2,5 @@
 host=review.openstack.org
 port=29418
 project=openstack/charm-odl-controller.git
+
+defaultbranch=stable/18.08

charm-helpers-tests.yaml

@@ -1,4 +1,4 @@
-repo: https://github.com/juju/charm-helpers
+repo: https://github.com/juju/charm-helpers@stable/18.08
 destination: tests/charmhelpers
 include:
     - contrib.amulet

tests/basic_deployment.py

@@ -67,7 +67,7 @@ class ODLControllerBasicDeployment(OpenStackAmuletDeployment):
     """Amulet tests on a basic OVS ODL deployment."""
 
     def __init__(self, series, openstack=None, source=None, git=False,
-                 stable=False, odl_version='helium'):
+                 stable=True, odl_version='helium'):
         """Deploy the entire test environment."""
         super(ODLControllerBasicDeployment, self).__init__(series, openstack,
                                                            source, stable)