Updates for stable branch 18.02 creation

Set default branch for git review/gerrit.

Switch amulet tests to stable.

Switch to using stable charm-helpers branch.

Change-Id: I6f8478c4647243d93515d740e714ea3b0637116d

.gitreview

@@ -2,3 +2,5 @@
 host=review.openstack.org
 port=29418
 project=openstack/charm-neutron-gateway.git
+
+defaultbranch=stable/18.02

charm-helpers-hooks.yaml

@@ -1,4 +1,4 @@
-repo: https://github.com/juju/charm-helpers
+repo: https://github.com/juju/charm-helpers@stable/18.02
 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/18.02
 destination: tests/charmhelpers
 include:
     - fetch

tests/basic_deployment.py

@@ -26,7 +26,7 @@ class NeutronGatewayBasicDeployment(OpenStackAmuletDeployment):
     """Amulet tests on a basic neutron-gateway deployment."""
 
     def __init__(self, series, openstack=None, source=None,
-                 stable=False):
+                 stable=True):
         """Deploy the entire test environment."""
         super(NeutronGatewayBasicDeployment, self).__init__(series, openstack,
                                                             source, stable)