engine: Rely on engine entities

The attach/detach networks to/from clusters commands
rely on entities provided by the clients, although
the entity could have other values than in the engine
db. In order to avoid such a discrepancy, the networks
are being retrieved internally.

Change-Id: I62f05855c09e5fdee862d18dcce59c343baeafa9
Signed-off-by: Moti Asayag <masayag@redhat.com>