Use unittest.mock instead of third party mock

Now that we no longer support py27, we can use the standard library
unittest.mock module instead of the third party mock lib. Most of this
is autogenerated, as described below, but there is one manual change
necessary:

nova/tests/functional/regressions/test_bug_1781286.py
  We need to avoid using 'fixtures.MockPatch' since fixtures is using
  'mock' (the library) under the hood and a call to 'mock.patch.stop'
  found in that test will now "stop" mocks from the wrong library. We
  have discussed making this configurable but the option proposed isn't
  that pretty [1] so this is better.

The remainder was auto-generated with the following (hacky) script, with
one or two manual tweaks after the fact:

  import glob

  for path in glob.glob('nova/tests/**/*.py', recursive=True):
      with open(path) as fh:
          lines = fh.readlines()
      if 'import mock\n' not in lines:
          continue
      import_group_found = False
      create_first_party_group = False
      for num, line in enumerate(lines):
          line = line.strip()
          if line.startswith('import ') or line.startswith('from '):
              tokens = line.split()
              for lib in (
                  'ddt', 'six', 'webob', 'fixtures', 'testtools'
                  'neutron', 'cinder', 'ironic', 'keystone', 'oslo',
              ):
                  if lib in tokens[1]:
                      create_first_party_group = True
                      break
              if create_first_party_group:
                  break
              import_group_found = True
          if not import_group_found:
              continue
          if line.startswith('import ') or line.startswith('from '):
              tokens = line.split()
              if tokens[1] > 'unittest':
                  break
              elif tokens[1] == 'unittest' and (
                  len(tokens) == 2 or tokens[4] > 'mock'
              ):
                  break
          elif not line:
              break
      if create_first_party_group:
          lines.insert(num, 'from unittest import mock\n\n')
      else:
          lines.insert(num, 'from unittest import mock\n')
      del lines[lines.index('import mock\n')]
      with open(path, 'w+') as fh:
          fh.writelines(lines)

Note that we cannot remove mock from our requirements files yet due to
importing pypowervm unit test code in nova unit tests. This library
still uses the mock lib, and since we are importing test code and that
lib (correctly) only declares mock in its test-requirements.txt, mock
would not otherwise be installed and would cause errors while loading
nova unit test code.

[1] https://github.com/testing-cabal/fixtures/pull/49

Change-Id: Id5b04cf2f6ca24af8e366d23f15cf0e5cac8e1cc
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

nova/tests/functional/test_instance_actions.py

@@ -12,7 +12,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import mock
+from unittest import mock
+
 from oslo_policy import policy as oslo_policy
 
 from nova import exception