Inherit MockWithCmp from Mock instead of MagicMock

This patch changes the base class for MockWithCmp and implements necessary
methods to compare instances, because mock.MagicMock in python 3 raises
NotImplemented exception on __lt__, __gt__, etc.

Change-Id: I56f774a5bc27726dae66acb70bf12bc618cb96fb
Fix "tests.unit.test_excutils" test with python 3.4

Mocking time.time() failed because python 3.4 did not pass "self" argument.

Change-Id: Icb9874250766eff20b8da0844a4fae33816088cb
Merge "Fix "tests.unit.test_excutils" test with python 3.4"
Add tests.unit.db.sqlalchemy.test_migrate to py33

This test module is running fine in Python 3.3, so it's being added to
tox.ini

Change-Id: I9e40e24ef081fb6299fa2e1b3dfe777f391ead21