docker: detect defaultNetworkMtu from default route

Docker-DCO-1.1-Signed-off-by: Johan Euphrosine <proppy@google.com> (github: google)
networkdriver: add ErrNoDefaultRoute

Docker-DCO-1.1-Signed-off-by: Johan Euphrosine <proppy@google.com> (github: google)
docker/config: update -mtu flag default

Docker-DCO-1.1-Signed-off-by: Johan Euphrosine <proppy@google.com> (github: google)
docs/cli: add mtu option

Docker-DCO-1.1-Signed-off-by: Johan Euphrosine <proppy@google.com> (github: google)
Merge pull request #3887 from proppy/detect-network-mtu

docker: detect defaultNetworkMtu from default route