Replaces mox with mock for ipmitool driver unit tests

OpenStack is moving unit tests from mox to mock for python3 compatibility.
This replaces mox with mock for ipmitool driver unit tests.

Fixes bug: #1229396

Change-Id: Ifbb9121fad78674002653502dd6f6c7f2bee3f5e
Merge "Replaces mox with mock for ipmitool driver unit tests"