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: I71ab3fcd308d4f1781d25feed96702d4d461dd81

.gitreview

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

charm-helpers-hooks.yaml

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

tests/basic_deployment.py

@@ -41,7 +41,7 @@ class OpenstackDashboardBasicDeployment(OpenStackAmuletDeployment):
     """Amulet tests on a basic openstack-dashboard deployment."""
 
     def __init__(self, series, openstack=None, source=None,
-                 stable=False):
+                 stable=True):
         """Deploy the entire test environment."""
         super(OpenstackDashboardBasicDeployment, self).__init__(series,
                                                                 openstack,