Consolidates TestCase and BaseTestCase

Most unit tests are derived from TestCase class of base.py.
A few unit tests are derived from BaseTestCase class of utils.py.
This patch consolidates these two classes into one and have all
the tests derived from a single base class which is TestCase class.
Hence, the ironic/tests/util.py is removed from this patch.

Change-Id: Iecc3352ef7e49524b202e6551a233692a2f6acea