Fix token response mock

The unit tests mock tokens from keystoneauth. One of the responses
being returned from a mocked keystoneauth method was returning a v2.0
token that didn't have the right expiration attribute of a v2.0
token. This was caught by keystoneauth 3.0.1, which underwent a
significant refactor and added a bunch of new functionality for
version discovery. Keystoneauth was expecting `expires` to be
in the token but instead the mock was using `expires_at`, which isn't
a valid v2.0 token attribute.

Closes-Bug: 1706538

Change-Id: Iea77ef54585d0b3480369e9f5df629e3f46f7e3b

troveclient/tests/test_shell.py

@@ -74,7 +74,7 @@ class ShellTest(testtools.TestCase):
     v2_auth_response = json.dumps({
         "access": {
             "token": {
-                "expires_at": "2020-01-01T00:00:10.000123Z",
+                "expires": "2020-01-01T00:00:10.000123Z",
                 "id": 'fakeToken',
                 "tenant": {
                     "id": uuid.uuid4().hex