Do not use contextlib.nested if only mock one function

[Guangya Liu (Jay Lau)]

There are some test cases in test_compute_mgr.py using contextlib.nested to mock up functions even there is only one function.

We should use mock.patch.object directly if only mock one function.

def test_init_instance_sets_building_error(self):
        with contextlib.nested( <<<<< No need use nested here
            mock.patch.object(self.compute, '_instance_update')
          ) as (
            _instance_update,
          ):

            instance = instance_obj.Instance(self.context)
            instance.uuid = 'foo'
            instance.vm_state = vm_states.BUILDING
            instance.task_state = None
            self.compute._init_instance(self.context, instance)
            call = mock.call(self.context, 'foo',
                             task_state=None,
                             vm_state=vm_states.ERROR)
            _instance_update.assert_has_calls([call])

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/63670

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/63670
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=8c359e0d0cf40b82b07ed1a30f6bfc00369892a6
Submitter: Jenkins
Branch: master

commit 8c359e0d0cf40b82b07ed1a30f6bfc00369892a6
Author: Jay Lau <email address hidden>
Date: Mon Dec 23 01:14:56 2013 -0500

    Do not use contextlib.nested if only mock one function

    There are some test cases in test_compute_mgr.py using contextlib.nested
    to mock up functions even there is only one function needs to be mocked.

    We should use mock.patch.object directly if only mock one function.

    Change-Id: I30f05d0dcb442d426358b673f0e638ace8b85e0e
    Closes-Bug: #1263602