ofproto-dpif: Implement translation of select groups.

ofproto-dpif: Implement translation of select groups.

Select bucket from those that are alive based on a hash of the destination
ethernet address of the packet.

Support for weights is proposed by a subsequent patch.

The selection is based on a hash of the destination ethernet
address of the flow. It should be possible to extend
this to cover a hash of user-specified elements of the flow.

Signed-off-by: Simon Horman <horms@verge.net.au>
[blp@nicira.com replaced bucket selection by "highest random weight"
 method]
Signed-off-by: Ben Pfaff <blp@nicira.com>