Use unittest.mock instead of third party mock

Now that OpenStack has moved to py36 and later runtimes, we no longer
need to use the third party mock library and can use the standard
library unittest.mock instead.

Change-Id: I100001fd0615cb55cbdbabbf0c6aa41b1729569c
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

lower-constraints.txt

@@ -44,7 +44,6 @@ linecache2==1.0.0
 MarkupSafe==1.0
 mccabe==0.2.1
 mistral-lib==1.2.0
-mock==2.0.0
 monotonic==1.4
 mox3==0.25.0
 msgpack==0.5.6

molecule-requirements.txt

@@ -6,6 +6,5 @@ pytest
 pytest-cov
 pytest-html
 pytest-xdist
-mock
 molecule>=2.22rc1,<3
 selinux  # MIT

tripleo_validations/tests/library/test_check_ironic_boot_config.py

@@ -13,12 +13,11 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+from unittest import mock
 
 import library.check_ironic_boot_config as validation
 from tripleo_validations.tests import base
 
-import mock
-
 UUIDs = [
     '13c319a4-7704-4b44-bb2e-501951879f96',
     '8201bb8e-be20-4a97-bcf4-91bcf7eeff86',