engine: adding "NetworkMigrationEnabled" to vdc_options.

Migration network is 3.3 feature.
Each cluster can have one migration network.
If a network is signed as a "migration network" the vm migration inside
the cluster will be done via this network.
If there is no  migration network the migration proccess will be done via
the managment network.

Change-Id: Ib553b8ac2c2015659c59c8cddfd2329530133b30
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>