Move veth management out of rattan core

[minhuw]

I finally realize that Rattan core should not take the responsibility of network device management. Instead, it learns the device it should operate on from the configuration file and doesn't care whether it's a physical NIC or an end of a veth pair.

It helps us integrate with docker and k8s more easily and, possibly more important, writing provisioning code in Rust (e.g., current Netlink code) is awkward. We should have written it in golang or Python. For the same reason, must we develop cli and rattan core in the same language?