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: I27ccd34f2337533b217d8c6f817a6d5a14e47abb

.zuul.yaml

@@ -1,4 +1,3 @@
 - project:
     templates:
-      - python35-charm-jobs
       - openstack-python3-ussuri-jobs

test-requirements.txt

@@ -13,11 +13,6 @@ setuptools<50.0.0  # https://github.com/pypa/setuptools/commit/04e3df22df840c6bb
 
 requests>=2.18.4
 
-# Newer mock seems to have some syntax which is newer than python3.5 (e.g.
-# f'{something}'
-mock>=1.2,<4.0.0; python_version < '3.6'
-mock>=1.2; python_version >= '3.6'
-
 stestr>=2.2.0
 
 # Dependency of stestr. Workaround for

unit_tests/test_actions.py

@@ -12,8 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
-from mock import patch
+from unittest import mock
+from unittest.mock import patch
 
 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
 
 # python-apt is not installed as part of test-requirements but is imported by
 # some charmhelpers modules so create a fake import.

unit_tests/test_ceilometer_contexts.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 import collections
-from mock import patch, MagicMock
+from unittest.mock import patch, MagicMock
 
 import ceilometer_contexts as contexts
 import ceilometer_utils as utils

unit_tests/test_ceilometer_hooks.py

@@ -16,7 +16,7 @@ import copy
 import os
 import sys
 
-from mock import patch, MagicMock, call, mock_open
+from unittest.mock import patch, MagicMock, call, mock_open
 
 # python-apt is not installed as part of test-requirements but is imported by
 # some charmhelpers modules so create a fake import.

unit_tests/test_ceilometer_utils.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from mock import patch, call, MagicMock
+from unittest.mock import patch, call, MagicMock
 
 import ceilometer_utils as utils
 

unit_tests/test_utils.py

@@ -19,7 +19,7 @@ import yaml
 import io
 
 from contextlib import contextmanager
-from mock import patch
+from unittest.mock import patch
 
 
 @contextmanager