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: Iab7c4a26d85c8a921a500165f99e0cdc23993940

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_cinder_backup_hooks.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
+from unittest import mock
 
 with mock.patch('cinder_backup_utils.register_configs'):
     import cinder_backup_hooks as hooks

unit_tests/test_utils.py

@@ -17,7 +17,7 @@ import unittest
 import os
 import yaml
 
-from mock import patch
+from unittest.mock import patch
 
 
 def load_config():