engine: Remove network from hosts when detached from cluster

When detaching a labeled network from a cluster, the label
became ineffective in that cluster in regards to the detached
network context. Therefore, the network should be removed
from all labeled hosts.

Change-Id: I513f5ce7332388ac44ddc1f05988f23d6531cb89
Bug-Url: https://bugzilla.redhat.com/1040586
Signed-off-by: Moti Asayag <masayag@redhat.com>