Use original_binding_levels when current binding_levels is not set
Use original_binding_levels when current binding_levels is not set

The binding_levels may be cleared when update_port is called (e.g. in
case of deleting a baremetal instance). In such a case, original_binding_levels
can be used to get required binding levels.

Change-Id: Ibab2c9d90023c52141ce0b9d57fdafaf0aadf8c9

openstack/networking-arista,stackforge/networking-arista,openstack/networking-arista