Minor fixes regarding nic.network change

Make LookupNetwork() return None in case target is None. This fixes
Issue 380. Rapi passes network=None and the lookup should not fail.

Make network client aware of new nic.network.gnt-network info shows
the IPs of each instance inside the network.  It parses nic.networks
field of QueryInstances() output, which now is a list of uuids an
not names.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Helga Velroyen <helgav@google.com>