Use mock to do unit tests for ipminative driver

The ipminative power driver uses mox to do the unit tests.
Now the OpenStack is moving to use mock instead of mox in order
to be ready for python 3. This patch changes the test cases for
ipminative driver from using mox to mock.

Change-Id: I9a337c4f38a364a64d61669b045a9610f9c59d42
Closes-Bug: 1231601