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: Ia06fa93973b153a265ddd0cb2538f5bdabf764aa

roles/validate-tempest/files/bugcheck/tests/test_bugcheck.py

@@ -16,7 +16,7 @@
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
-import mock
+from unittest import mock
 import os
 import tempfile
 import unittest

roles/validate-tempest/files/tempestmail/tests/test_tempestmail.py

@@ -17,7 +17,7 @@
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 import datetime
-import mock
+from unittest import mock
 import re
 import tempfile
 import unittest