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: Id49c895492a7f6cd101ccbff2e7645b67e0f56e8

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_spark/tests/unit/plugins/spark/test_config_helper.py

@@ -16,7 +16,7 @@
 import copy
 import xml.dom.minidom as xml
 
-import mock
+from unittest import mock
 
 from sahara.plugins import swift_helper as swift
 from sahara.plugins import utils

sahara_plugin_spark/tests/unit/plugins/spark/test_plugin.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
+
 import testtools
 
 from sahara.plugins import base as pb

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