OpenStackCharm class reset adapters_class to None

The base OpenStackCharm class sets the adapters_class, but unfortunately
in the same function, it then sets it to None.  Although this was the
default behaviour, it was changed so that there was a default
adapters_class.  Unfortunately, this was missed.

This patchset fixes that.

Change-Id: If4e2b1abe33106657f0638179c86c0d619240e98

charms_openstack/charm.py

@@ -544,7 +544,6 @@ class OpenStackCharm(object):
     configuration_class = os_adapters.ConfigurationAdapter
 
     ha_resources = []
-    adapters_class = None
     HAPROXY_CONF = '/etc/haproxy/haproxy.cfg'
     MEMCACHE_CONF = '/etc/memcached.conf'
     package_codenames = {}

unit_tests/test_charms_openstack_charm.py

@@ -454,7 +454,9 @@ class TestOpenStackCharm__init__(BaseOpenStackCharmTest):
     def test_empty_init_args(self):
         target = chm.OpenStackCharm()
         self.assertIsNone(target.release)
-        self.assertIsNone(target.adapters_instance)
+        # we expect target.adapters_instance to not be None as
+        # target.adapters_class is not None as a default
+        self.assertIsNotNone(target.adapters_instance)
         # from mocked hookenv.config()
         self.assertEqual(target.config, TEST_CONFIG)