Add locking to _NetworkQuery

Make use of _GetNames() to get the final network UUIDs.
Get networks info after having aquired the locks. Refactor
the code to use new local variables:

 - "network_uuids" returned from _GetNames()
 - "all_networks" that are the latest network info
 - "name_to_uuid" used for getting network UUID out of NIC's network

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>