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: Ibf4c4fd66bb5b5340af0a4959f86524229199d4e

.gitreview

@@ -2,3 +2,5 @@
 host=review.openstack.org
 port=29418
 project=openstack/charm-percona-cluster.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:
     - contrib.amulet

tests/basic_deployment.py

@@ -21,7 +21,7 @@ class BasicDeployment(OpenStackAmuletDeployment):
     utils = AmuletUtils()
 
     def __init__(self, vip=None, units=1, series="trusty", openstack=None,
-                 source=None, stable=False):
+                 source=None, stable=True):
         super(BasicDeployment, self).__init__(series, openstack, source,
                                               stable)
         self.units = units