DNS fix: do not traceback if unsupported records are in LDAP
Show records which are unsupported, if they are in LDAP.
Those records are not editable, and web UI doesnt show them.
Fixes traceback caused by --structured option
Ticket: https://fedorahosted.org/freeipa/ticket/4930
Reviewed-By: Petr Spacek <pspacek@redhat.com>