DNS install: Ensure that DNS servers container exists
    
    during DNS installation it is assumed that the cn=servers,cn=dns container is
    always present in LDAP backend when migrating DNS server info to LDAP.
    
    This may not always be the case (e.g. when a new replica is set up against
    older master) so the code must take additional steps to ensure this container
    is present.
    
    https://fedorahosted.org/freeipa/ticket/6083
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>