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 modify file: heatclient/tests/unit/test_common_http.py

Change-Id: I220fabedb8e9fcc8eeea9d5e98b3ddeed68f8a43
Implements: bp remove-mox3-in-heatclient

openstack/python-heatclient