SCTX-1723: NS-SDX interface has name eth3_rename

To fix issue described in SCTX-1723, interface-rename.py is changed to
not write any rules into 60-net.rules. This causes all nics on bootup to
be renamed to side-<num>-eth<num> by udev. Some of the warning messages
from logic.py are changed; finding side-$RANDOM-eth$N is now the norm
rather than the exception.