Updates for stable branch creation

Set default branch for git review/gerrit.

Switch tests to stable.

Switch to using stable charm-helpers branch.

Switch to using stable charm.openstack branch.

Change-Id: Ie6cc091eae5b9336d6e12afbd6dbf582dbb3c0b5

.gitreview

@@ -2,3 +2,5 @@
 host=review.opendev.org
 port=29418
 project=openstack/charm-percona-cluster.git
+
+defaultbranch=stable/20.08

charm-helpers-hooks.yaml

@@ -1,4 +1,4 @@
-repo: https://github.com/juju/charm-helpers
+repo: https://github.com/juju/charm-helpers@stable/20.08
 destination: charmhelpers
 include:
     - core

tests/bundles/bionic.yaml

@@ -8,7 +8,7 @@ applications:
     charm: ../../../percona-cluster
     num_units: 1
   keystone:
-    charm: cs:~openstack-charmers-next/keystone
+    charm: cs:~openstack-charmers/keystone
     num_units: 1
     options:
       token-expiration: 60

tests/bundles/overlays/ha.yaml

@@ -9,7 +9,7 @@ applications:
       vip: {{ TEST_VIP00 }}
       min-cluster-size: 3
   hacluster:
-    charm: cs:~openstack-charmers-next/hacluster
+    charm: cs:~openstack-charmers/hacluster
     num_units: 0
     options:
       cluster_count: 3

tests/bundles/xenial.yaml

@@ -8,7 +8,7 @@ applications:
     charm: ../../../percona-cluster
     num_units: 1
   keystone:
-    charm: cs:~openstack-charmers-next/keystone
+    charm: cs:~openstack-charmers/keystone
     num_units: 1
     options:
       token-expiration: 60