Bug-1399: Added Linkstate attribute type configuration

-IANA has issued an early allocation for the BGP Linkstate path attribute (=29)
-type is configurable to preserve backwards compatibility

Change-Id: I24adfea7abfa80bc897415317c8cd0f856d6b6f9
Signed-off-by: Milos Fabian <milfabia@cisco.com>
Merge "Bug-1399: Added Linkstate attribute type configuration"