Additional tests for safe parsing with minidom

For nova, forbid_dtd is going to be true always, however
if someone picks up this code and tries forbid_dtd = False
then the existing code is not good enough. we need to protect
against external entities/dtd and not allow notations as well.
Added a few more handlers and test cases to cover that use
case.

Change-Id: If50f690e015f2bf837b403edf552e35d7af8c907