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: Ib8c30a3357c57f9c127b94d777610d602454e0ef

lower-constraints.txt

@@ -57,7 +57,6 @@ Mako==1.0.7
 MarkupSafe==1.1.0
 mccabe==0.5.3
 microversion-parse==0.2.1
-mock==2.0.0
 monotonic==1.5
 mox3==0.26.0
 msgpack==0.6.1

sahara_plugin_storm/tests/unit/base.py

@@ -13,7 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
+from unittest import mock
+
 from oslotest import base
 
 from sahara import context

sahara_plugin_storm/tests/unit/plugins/storm/test_plugin.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 sahara.plugins import base as pb
 from sahara.plugins import conductor

test-requirements.txt

@@ -9,7 +9,6 @@ bashate>=0.5.1 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
 doc8>=0.6.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
-mock>=2.0.0 # BSD
 oslotest>=3.2.0 # Apache-2.0
 stestr>=1.0.0 # Apache-2.0
 pylint==1.4.5 # GPLv2