ansible_freeipa_module: Fix ansible-test fake execution test findings

All imports that are only available after installing IPA need to be in a
try exception clause to be able to pass the fake execution test. The old
workaround "if 'ansible.executor' in sys.modules:" is not working with
this test anymore.

If the imports can not be done, all used and needed attributes are
defines with the value None.

A check has been added to IPAAnsibleModule.__init__ to make sure that it
fails if the imports have not been done successfully.