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: Ibddd954daaf3cd008985ff27cd95dfe649476555

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

@@ -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_ceilometer_hooks.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 import json
-from mock import patch
+from unittest.mock import patch
 
 import ceilometer_utils
 

unit_tests/test_ceilometer_utils.py

@@ -14,7 +14,7 @@
 
 import sys
 
-from mock import MagicMock, patch
+from unittest.mock import MagicMock, patch
 
 # 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_utils.py

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