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.

Change-Id: I7e645c6fcc302ed152c57e997c32ebe658e4d0bd

test-requirements.txt

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

unit_tests/test_cinder_hooks.py

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

unit_tests/test_cinder_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
+from unittest.mock import patch, call
 import os
 import cinder_utils as cinder_utils
 

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():