restapi: Attach network to cluster by ID

The patch resolves the network which is a candidate
to be attached to a cluster or to be updated within
a cluster by queries all of the networks on DC level
instead for all the networks which are currently
attached to the cluster.

Change-Id: I2293cccf3d88aea384bd61ae75d3c8d5f41408a9
Bug-Url: https://bugzilla.redhat.com/961046
Signed-off-by: Moti Asayag <masayag@redhat.com>