Isolate test from environment variables

One test ('test_get_client_all_creds') will fail if any of the typical
Openstack environment variables have been set (eg OS_PASSWORD).

Change-Id: I982f526168be7a5a07b5cd48af522f7f9bc74f2e
Closes-bug: #1376247

glance/tests/unit/test_image_cache_client.py

@@ -78,13 +78,19 @@ class GetClientTestCase(utils.BaseTestCase):
     def setUp(self):
         super(GetClientTestCase, self).setUp()
         self.host = 'test_host'
+        self.env = os.environ.copy()
+        os.environ.clear()
+
+    def tearDown(self):
+        os.environ = self.env
+        super(GetClientTestCase, self).tearDown()
 
     def test_get_client_host_only(self):
         expected_creds = {
             'username': None,
             'password': None,
             'tenant': None,
-            'auth_url': os.getenv('OS_AUTH_URL'),
+            'auth_url': None,
             'strategy': 'noauth',
             'region': None
         }