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: I12800928a03a63e4c5028b6bbffda1b89dd9ff24

.gitreview

@@ -2,3 +2,5 @@
 host=review.opendev.org
 port=29418
 project=openstack/charm-vault.git
+
+defaultbranch=stable/19.07

src/tests/bundles/xenial-ha-mysql.yaml

@@ -7,9 +7,9 @@ services:
     options:
         vip: 'ADD YOUR VIP HERE'
   hacluster:
-    charm: cs:~openstack-charmers-next/hacluster
+    charm: cs:hacluster
   mysql:
-    charm: cs:~openstack-charmers-next/percona-cluster
+    charm: cs:percona-cluster
     num_units: 1
   easyrsa:
     charm: cs:~containers/easyrsa

src/tests/bundles/xenial-mysql.yaml

@@ -5,21 +5,21 @@ services:
     series: xenial
     charm: vault
   mysql:
-    charm: cs:~openstack-charmers-next/percona-cluster
+    charm: cs:percona-cluster
     num_units: 1
   keystone:
-    charm: cs:~openstack-charmers-next/keystone
+    charm: cs:keystone
     num_units: 1
     options:
       admin-password: openstack
       openstack-origin: cloud:xenial-queens
   ceph-mon:
-    charm: cs:~openstack-charmers-next/ceph-mon
+    charm: cs:ceph-mon
     num_units: 3
     options:
       source: cloud:xenial-queens
   ceph-osd:
-    charm: cs:~openstack-charmers-next/ceph-osd
+    charm: cs:ceph-osd
     num_units: 3
     options:
       source: cloud:xenial-queens