py27 tox tests fail with a TypeError

[Ryan Petrello]

Currently, tests run via `tox -epy27` are failing with a TypeError:

An example of the full traceback of the failure:

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):
      File "designate/tests/unit/test_backend/test_designate.py", line 78, in setUp
        self.backend = impl_designate.DesignateBackend(self.target)
      File "designate/backend/impl_designate.py", line 42, in __init__
        super(DesignateBackend, self).__init__(target)
      File "designate/backend/base.py", line 49, in __init__
        self.admin_context = DesignateContext.get_admin_context()
      File "designate/context.py", line 138, in get_admin_context
        return cls(None, **kwargs)
      File "designate/context.py", line 45, in __init__
        super(DesignateContext, self).__init__(
    TypeError: must be type, not MagicMock

======
Totals
======
Ran: 1249 tests in 81.0000 sec.
 - Passed: 1238
 - Skipped: 3
 - Expected Fail: 4
 - Unexpected Success: 0
 - Failed: 4