Low: IPaddr2: support nic:iflabel format in nic parameter

IPaddr2 in 3.9.4 (re-written to use findif.sh) does not accept nic=eth0:iflabel.
Although it is considered deprecated as per the meta-data description of nic,
it would be good to continue to support it for the backward compatibility.

ref.
http://www.gossamer-threads.com/lists/linuxha/dev/83954#83954