Cert: Cleanup gd/cert Python libraries

* Rename gd_base_test_facade_only to gd_base_test and its enclosing
  class GdBaseTestClass
* Moved OS utilities to an unified file cert/os_utils.py
* Sort imports according to PEP 8

Test: source gd/cert/run --host
Change-Id: I0d308975de72bcfc0a5aec89776137a104b76387