Allow user to set container internal network interface name

Hi,

I found myself needing more than just one additional interface for my containers, because my network configuration of containers reflects a production system with several independent VLAN.

I modified pipework to allow the user to (optionally) set the container internal interface. The optional parameter is positional and has to follow after IFNAME on the command line. Further, I added a verbose options to print relevant (to me) information during pipework works. 

The command line parsing is not very nice, but I tried to stay as close to the original syntax to not break any existing usage of pipework.

I'd be happy, if you accept my pull request.

Bye,
Lukas