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: Ib3847b49f41c054b998072d0fbd0648b5293af5b

sahara_tests/unit/scenario/test_base.py

@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
+from unittest import mock
 from saharaclient.api import cluster_templates
 from saharaclient.api import clusters
 from saharaclient.api import data_sources

sahara_tests/unit/scenario/test_runner.py

@@ -18,7 +18,7 @@ import sys
 
 from jsonschema import exceptions
 import pkg_resources as pkg
-import mock
+from unittest import mock
 import testtools
 
 from sahara_tests import version

sahara_tests/unit/scenario/test_utils.py

@@ -13,7 +13,7 @@
 
 import os
 
-import mock
+from unittest import mock
 import testtools
 
 from sahara_tests.scenario import utils

test-requirements.txt

@@ -8,5 +8,4 @@ bandit!=1.6.0
 bashate>=0.2 # Apache-2.0
 coverage>=3.6 # Apache-2.0
 doc8 # Apache-2.0
-mock>=1.2 # BSD
 pylint==1.4.5 # GNU GPL v2