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>