engine: Attached network to host for unused labeled network

The engine should support configuring a labeled network
on hosts, if the network was already labeled, but never
configured on any host.

Change-Id: I646dd1901b2e75b1b6bebc5562c9cc2e8377425d
Bug-Url: https://bugzilla.redhat.com/1074258
Signed-off-by: Moti Asayag <masayag@redhat.com>