Replace mox3 with mock
Replace mox3 with mock

In heatclient, some unit tests still use mox3,
which will make the test speed much slow, so I decide to remove it.
The first file: heatclient/tests/unit/test_events.py

Change-Id: Ife264461d64b0cb89b251a0aefe0e5f255b39ce3
Implements: bp remove-mox3-in-heatclient

openstack/python-heatclient