Rename __unicode__ to __str__ in classes which define it and use the
six.python_2_unicode_compatible decorator on them to make them compatible with
both Python 2 and 3.
Additional changes were required for the ipapython.dnsutil.DNSName class,
because it defined both __str__ and __unicode__.
Reviewed-By: Petr Viktorin <pviktori@redhat.com>