Removes vlan from BridgePort.

The Vlan ID is not part of the original bridge port attributes. The
current CLI shows "VLAN: -1" for a VxLAN port when we list ports for a
bridge with a VxLAN port, which is useless and misleading. This patch
removes the attribute.

**Old CLI output**
midonet> bridge bridge5 list port
port port0 device bridge5 state up vlan -1 management-ip 119.15.112.22 vni 12304763

**New CLI output**
midonet> bridge bridge5 list port
port port0 device bridge5 state up management-ip 119.15.112.22 vni 12304763

Fix MN-2386
I'm not really familiar with Python conventions. Should this be in all-caps?