only check for netbios name when LDAP backend is connected
This is to prevent errors due to non-existent LDAP connection such as
when installing first IPA master.
https://fedorahosted.org/freeipa/ticket/6630
Reviewed-By: Martin Basti <mbasti@redhat.com>
Reviewed-By: Jan Cholasta <jcholast@redhat.com>