dns: check if container exists using ldapi
    
    Previously an adhoc connection was established for checking if
    dns(sec) container exists. A simple or external bind was used.
    Instead, always connect with ldapi through api.Backend.ldap2.
    
    https://fedorahosted.org/freeipa/ticket/6461
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>