Add support for virtual IPoIB interfaces (ipoib subinterface) & partition keys

Add support for virtual ipoib interfaces.  These are similar to ethernet macvlan.  It prevents the device being moved only into a single container.
Also added is support for Infiniband Partition Keys (which are similar to the ethernet VLAN).

Use like the following:

    pipework ib0 $CONTAINERID 10.10.10.1/24
    pipework ib0 $CONTAINERID 10.10.10.1/24 @8001  # for PKEY 8001 which must be set up in your subnet manager.

I've also updated the Readme for reflect the changes.
Is github formatter playing tricks on me, or the indentation is slightly inconsistent here?

(Yeah, I know, I'm nitpicking; but it's just to prove that I'm paying attention... Or trying :-))