network: GetDefaultNetworkMtu from eth0

get the default -mtu flag value from eth0's MTU, fallback on `1500` in case of error.

Not a fan of hardwiring `eth0` there, let me know if there is a better way (tm).

Docker-DCO-1.0-Signed-off-by: Johan Euphrosine proppy@google.com (github: proppy)