Move charm-cinder-backup-swift-proxy to new hacking 4.0.0

This patch changes old assert checks to new optimal assert
checks provided by hacking plugin in release 4.0.0.
Please refer [1] for optimal assert checks details.

The patch modifies following,
 * Changes assertTrue/False(A in/not in B, message) to assertIn/NotIn(A, B, message).

[1] https://docs.openstack.org/releasenotes/hacking/unreleased.html#relnotes-4-0-0

Change-Id: Ie95e429ac63ecbc61a728f52c32171ba3133414c

unit_tests/test_cinder_backup_handlers.py

@@ -108,4 +108,4 @@ class TestCinderBackupSwiftHandlers(unittest.TestCase):
 
             for f, args in t.items():
                 # check that function is in patterns
-                self.assertTrue(f in p.keys())
+                self.assertIn(f, p.keys())