CLOUDSTACK-4984: decrement MAX_VXLAN_VNI to be aligned with Linux kernel

Linux vxlan interface doesn't accept VNI:16777215 now.

Signed-off-by: Toshiaki Hatano <haeena@haeena.net>
CLOUDSTACK-4932: bugfix : listNetworks API doesn't return VLAN ID (VNI)

Signed-off-by: ynojima <mail@ynojima.net>
Signed-off-by: Toshiaki Hatano <haeena@haeena.net>