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: I2915f5b7b31752a6c79027c9b1a73e6ac1e93ed1

tests/bundles/disco-stein.yaml

@@ -9,43 +9,43 @@ applications:
     options:
       osd-devices: '/srv/ceph /dev/test-non-existent'
   ceph-mon:
-    charm: cs:~openstack-charmers-next/ceph-mon
+    charm: cs:ceph-mon
     num_units: 3
     options:
       monitor-count: '3'
       auth-supported: 'none'
   percona-cluster:
-    charm: cs:~openstack-charmers-next/percona-cluster
+    charm: cs:percona-cluster
     num_units: 1
     options:
       dataset-size: 25%
       max-connections: 1000
   rabbitmq-server:
-    charm: cs:~openstack-charmers-next/rabbitmq-server
+    charm: cs:rabbitmq-server
     num_units: 1
   keystone:
     expose: True
-    charm: cs:~openstack-charmers-next/keystone
+    charm: cs:keystone
     num_units: 1
   nova-compute:
-    charm: cs:~openstack-charmers-next/nova-compute
+    charm: cs:nova-compute
     num_units: 1
   glance:
     expose: True
-    charm: cs:~openstack-charmers-next/glance
+    charm: cs:glance
     num_units: 1
   cinder:
     expose: True
-    charm: cs:~openstack-charmers-next/cinder
+    charm: cs:cinder
     num_units: 1
     options:
       block-device: 'None'
       glance-api-version: '2'
   cinder-ceph:
-    charm: cs:~openstack-charmers-next/cinder-ceph
+    charm: cs:cinder-ceph
   nova-cloud-controller:
     expose: True
-    charm: cs:~openstack-charmers-next/nova-cloud-controller
+    charm: cs:nova-cloud-controller
     num_units: 1
 relations:
 - - nova-compute:amqp