Add Infiniband ipoib support. Suppress ugly 'which' output.

Suppress: "which: no ovs-vsctl in (/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:/opt/bin:/mnt/host/source/chromite/bin)"  when openvswitch is not installed.

Add support for Infiniband ipoib interfaces which doesn't work without this change.
The specific error returned was "RTNETLINK answers: Invalid argument"

ipoib interfaces default to ib1 in the guest unless -i option used.  Must not begin with "eth".
Should work with SR-IOV ipoib devices as well.

Do you mean: "Note: if no container interface name was specified, pipework will default it to eb1" ?