Use unittest.mock instead of mock

The mock third party library was needed for mock support in py2
runtimes. Since we now only support py36 and later, we can use the
standard lib unittest.mock module instead.

Note that https://github.com/openstack/charms.openstack is used during tests
and he need `mock`, unfortunatelly it doesn't declare `mock` in its
requirements so it retrieve mock from other charm project (cross dependency).
So we depend on charms.openstack first and when
Ib1ed5b598a52375e29e247db9ab4786df5b6d142 will be merged then CI
will pass without errors.

Depends-On: Ib1ed5b598a52375e29e247db9ab4786df5b6d142
Change-Id: Ic61aac57ab757e555be0eef818052c58b8cf03dd

test-requirements.txt

@@ -9,7 +9,6 @@
 #
 charm-tools>=2.4.4
 requests>=2.18.4
-mock>=1.2
 flake8>=2.2.4,<=2.4.1
 stestr>=2.2.0
 coverage>=4.5.2

unit_tests/test_actions.py

@@ -14,7 +14,7 @@
 
 import os
 
-import mock
+from unittest import mock
 
 from test_utils import CharmTestCase
 

unit_tests/test_actions_openstack_upgrade.py

@@ -15,7 +15,7 @@
 import os
 import sys
 
-from mock import patch, MagicMock
+from unittest.mock import patch, MagicMock
 
 os.environ['JUJU_UNIT_NAME'] = 'heat'
 

unit_tests/test_heat_context.py

@@ -15,7 +15,7 @@
 import charmhelpers
 import heat_context
 import json
-from mock import patch
+from unittest.mock import patch
 from test_utils import CharmTestCase
 
 TO_PATCH = [

unit_tests/test_heat_relations.py

@@ -15,7 +15,7 @@
 import os.path
 import sys
 
-from mock import call, patch, MagicMock
+from unittest.mock import call, patch, MagicMock
 from test_utils import CharmTestCase, patch_open
 
 # python-apt is not installed as part of test-requirements but is imported by

unit_tests/test_heat_utils.py

@@ -14,7 +14,7 @@
 
 from copy import deepcopy
 from collections import OrderedDict
-from mock import patch, MagicMock, call
+from unittest.mock import patch, MagicMock, call
 from test_utils import CharmTestCase
 
 from charmhelpers.core import hookenv

unit_tests/test_utils.py

@@ -18,7 +18,7 @@ import unittest
 import yaml
 
 from contextlib import contextmanager
-from mock import patch, MagicMock
+from unittest.mock import patch, MagicMock
 
 
 def load_config():