[sig][upgrades] job done

Hi All After this mornings PTG session, I'm pleased to propose that we formally end the Upgrades SIG. That’s a “pleased” because we feel that our job as a SIG is done! Upgrades in OpenStack are no longer a "special interest"; they are now an integral part of the philosophy of projects within the OpenStack ecosystem and although there are probably still some rough edges, we don’t think we need a SIG to drive this area forward any longer. So thanks for all of the war stories, best practice discussion and general upgrade related conversation over the Forums, PTG’s and Summits over the last few years - it's been fun! Regards James