webadmin: add copy of migration property to NetworkCluster cloner.

"migration" is a property that was added to NetworkCluster business entity.
The cloner of NetworkCluster should clone also this entity.

Change-Id: I29d61c9cea1cc6d6269352297fd22f16d988450f
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>