use the methods of the parent class to retrieve CIFS kerberos keys
    
    adtrustinstance will now use parent's methods to retrieve keys for CIFS
    principal. Since the keys are appended to the host keytab
    (/etc/krb5.keytab) we need to make sure that only the stale CIFS keys
    are purged from the file and that we do not re-set its ownership.
    
    https://fedorahosted.org/freeipa/ticket/6638
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>