Mock pyghmi lib in unit tests if not present

Mock the pyghmi library from within ironic/tests/drivers so that
test_ipminative.py can import ironic/drivers/ipminative and run the unit
tests on it, even when the external library is not present. Also
corrects pep8 issue in ipminative by grouping the imports.

Change-Id: I870724bf48346857b1170463fcc7d22c08aca3bd
Merge "Mock pyghmi lib in unit tests if not present"