get_nodes: do not force net_3_name to be defined

Not defining the net_3_name will result in the jinja template
failing silently and none of the variables are set. The failure
happens because net_3_name is undefined and
item.0.info.addresses[net_3_name] cannot be evaluated.

Change-Id: I3555714c81112ca1ba372782420f36867b682355