Make mock import python 2.7 compatible

In python3.x mock is now part of unittest library.
For being still compatible with python 2.7 we need to
catch import error when trying to import unittest.mock.

Also add an __init__.py file in validations_common/library,
to allow all unit tests to import successfully.

This is similar to [1] for validations-libs.

[1]- https://review.opendev.org/716597

Change-Id: I6a81dc2640e9c4af3a07158bea95ebaec9fd466b

validations_common/tests/library/test_check_package_update.py

@@ -12,8 +12,12 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-from unittest.mock import MagicMock
-from unittest.mock import patch
+try:
+    from unittest.mock import MagicMock
+    from unittest.mock import patch
+except ImportError:
+    from mock import MagicMock
+    from mock import patch
 
 from validations_common.library.check_package_update import check_update
 from validations_common.library.check_package_update import get_package_details

validations_common/tests/library/test_reportentry.py

@@ -23,7 +23,10 @@ Tests for `reportentry` module.
 import validations_common.library.reportentry as validation
 from validations_common.tests import base
 
-from unittest import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 
 reason = "Reason #1"
 recommendation = ['Recommendation #1']