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.

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('watcher/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)

Co-Authored-By: Sean McGinnis <sean.mcginnis@gmail.com>

Change-Id: Icf35d3a6c10c529e07d1a4edaa36f504e5bf553a

test-requirements.txt

@@ -6,7 +6,6 @@ coverage>=4.5.1 # Apache-2.0
 doc8>=0.8.0 # Apache-2.0
 freezegun>=0.3.10 # Apache-2.0
 hacking>=3.0.1,<3.1.0 # Apache-2.0
-mock>=2.0.0 # BSD
 oslotest>=3.3.0 # Apache-2.0
 os-testr>=1.0.0 # Apache-2.0
 testscenarios>=0.5.0 # Apache-2.0/BSD