Correctly mock keystoneclient.Client.auto_token property with Mox

Recent Mox doesn't stub out class properties any more (check
https://code.google.com/p/pymox/issues/detail?id=11), so we have to do
that manually. Avoids the following error:

FAIL: test_get_default_role
(openstack_dashboard.test.api_tests.keystone_tests.RoleAPITests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/root/Projects/openstack/horizon/openstack_dashboard/test/api_tests/keystone_tests.py", line 76, in test_get_default_role
    keystoneclient = self.stub_keystoneclient()
  File "/root/Projects/openstack/horizon/openstack_dashboard/test/helpers.py", line 280, in stub_keystoneclient
    self.keystoneclient = self.mox.CreateMock(keystone_client.Client)
  File "/usr/lib/python2.7/site-packages/mox.py", line 258, in CreateMock
    new_mock = MockObject(class_to_mock, attrs=attrs)
  File "/usr/lib/python2.7/site-packages/mox.py", line 556, in __init__
    attr = getattr(class_to_mock, method)
  File "/usr/lib/python2.7/site-packages/mox.py", line 608, in __getattr__
    raise UnknownMethodCallError(name)
UnknownMethodCallError: Method called is not a member of the object: Method called is not a member of the object: auth_token
>>  raise UnknownMethodCallError('auth_token')

Change-Id: I54fd0de298dc66344470147d9bcec6b62baf8297
(cherry picked from commit 1e867700415b17c29056fb55069b6a3e2b22cf1b)
Fixes bug 1161901