restapi: Reduce backend calls for DC network list

In case user provides both ID and Name for the network
to be attached to a cluster, same networks list
returned by querying the backend could be reused.

Change-Id: Icf65412ace7b5e2da44af9d9288362ab08505129
Signed-off-by: Moti Asayag <masayag@redhat.com>