totemconfig: Add support for knet_mtu

totem.knet_mtu is new configuration option which allows setting
of automatic or manual knet MTU.

Also reload of totem.knet_pmtud_interval is fixed now, so it works when
key is deleted (and set back default value).

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>