Send along the IP address of the plab node in the return string from
the 'plabconfig' command.  We can't trust that the node will have
a resolvable hostname (or have working DNS even..), so slap down the IP
we have on record in the DB into a file.  This will be used by the
event proxy, which needs to know the node's routable IP in order to
subscripe to elvind on ops properly.