Discover the veth endpoint name instead of using default eth0

[gntouts]

Right now we use a default value (eth0) for the name of the veth container endpoint inside the netns. We need to discover that by examining the flags of all interfaces in the netns.

[gntouts]

A good starting point is the kata-containers approach: https://github.com/kata-containers/kata-containers/blob/3b2fb6a604a624f1ec0c21a85d6e4182344aac4c/src/runtime/virtcontainers/network_linux.go#L337