ofproto: Keep inserting buckets into a group from changing group type.

ofproto: Keep inserting buckets into a group from changing group type.

The "insert buckets" and "delete buckets" operations on a group should not
change the group's type or properties, but the implementation did this by
mistake.  This fixes the problem.

Reported-by: shivani dommeti <shivani.dommeti@gmail.com>
Tested-by: shivani dommeti <shivani.dommeti@gmail.com>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-discuss/2017-December/045830.html
Signed-off-by: Ben Pfaff <blp@ovn.org>