podnetwork: support jumbo frames

[yoheiueda]

Currently, jumbo frames (MTU size larger than 1,500) are not supported, since the current implementation of peer pods restricts a maximum MTU size to be not greater than 1450. (#68)

EKS supports jumbo frames (MTU 9001) as reported in #1966 (comment), but peer pods can't get benefits from jumbo frames.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/network_mtu.html

To support jumbo frames in peer pods, MTU of VXLAN devices need to be adjusted.