Introduce Flow-aware Bandwidth constraint

For intents other than point-to-point, amount of bandwidth to allocate is not straight forward.

e.g., When 2G stream and 3G stream flow into a Device, should egress port allocate 3G or 5G?

Allocation strategy today with ONOS-5808 is similar to using the max of all incoming flows. (There's implicit assumption all incoming BW is the same)

There should be a constraint so that strategy which takes a sum of ingress BW will be allocated for the egress.

See discussion in Change-Id: I2eb3c16efbce619db6d0d2ba415a35752a61ece4