Add tool for easy mocking.

[kramarz]

Not sure if it is right place for this issue. Example from our tests:

identity_endpoint = mock.Mock()
    identity_endpoint.post.return_value = request_factory.ApiResponse(201, {"data": identity})

    def mock_endpoint(name):
        mapping = [
            (re.compile('^identities$'), identity_endpoint),
        ]
        for regexp, endpoint in mapping:
            if regexp.match(name):
                return endpoint
        else:
            return mock.Mock()
         api = mock.MagicMock()
         api.endpoint.side_effect = mock_endpoint