Update to use interface based addressing instead of discovering IPs

[MaerF0x0]

We can now use "eth0" or whichever interface for the

--swarm-opt advertise=eth0:3376

and

--engine-opt cluster-advertise:2376

[dweomer]

I was aware of these but chose to use IP addresses instead as that was the most direct. This project is meant to be instructional/exploratory as well as portable across all supported virtualization infrastructures supported by Docker Machine. When using AWS the concept of an external interface isn't valid because inbound connections are NATd in. This breaks the parity (where an address has an interface) when trying to understand the cluster implementation differences across Docker Machine drivers with respect to internal/external IP addresses. If you would still like to see this I am accepting pull requests =)

[MaerF0x0]

Awesome, I'll see if I can pull something together for this and the other issue.