ipahostgroup: Fix changed flag, support IPA 4.6 on RHEL-7, new test cases

The changed flag returned by ipahostgroup calls have not always been correct.
The use of the module with IPA version 4.6 on RHEL-7 resulted in encoding
errors. All this has been fixed.

Addtitionally new test cases have been added to make sure that the issues
are solved.