Fixed manager.py to support multinode test on vnic_port

1. _create_port moved to ScenarioTest class
2. Fixed getting networks if not passed

Co-Authored-By: Moshe Levi <moshele@mellanox.com>
Closes-Bug: 1598792

Change-Id: Ib43d9d1faba00be122f8a29b43b07b5b74985902
1 file changed