Make OpenFlow 1.2+ role replies return the generation ID.

Make OpenFlow 1.2+ role replies return the generation ID.

OpenFlow extensibility working group issue EXT-272 clarifies the use of
the generation_id in role reply messages as used for the current generation
ID or all-1-bits if there is no current generation ID.  This commit
implements EXT-272 in Open vSwitch.

Unfortunately the full text of EXT-272 is not available freely online.
(The "open" part of the Open Networking Foundation is the network, not
the foundation)

EXT-272.
CC: Jarno Rajahalme <jarno.rajahalme@nsn.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>