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: Iaed0b79e2b0bcab41466794601cee833b27e1e19

.gitreview

@@ -2,3 +2,5 @@
 host=review.openstack.org
 port=29418
 project=openstack/charm-neutron-api.git
+
+defaultbranch=stable/19.04

charm-helpers-hooks.yaml

@@ -1,4 +1,4 @@
-repo: https://github.com/juju/charm-helpers
+repo: https://github.com/juju/charm-helpers@stable/19.04
 destination: hooks/charmhelpers
 include:
     - core

tests/basic_deployment.py

@@ -32,7 +32,7 @@ class NeutronAPIBasicDeployment(OpenStackAmuletDeployment):
     """Amulet tests on a basic neutron-api deployment."""
 
     def __init__(self, series, openstack=None, source=None,
-                 stable=False):
+                 stable=True):
         """Deploy the entire test environment."""
         super(NeutronAPIBasicDeployment, self).__init__(series, openstack,
                                                         source, stable)