[Rdo-list] Update packstack core list

Hi, I've looked at packstack core-list [1] and I suggest we revisit to keep only active contributors [2] in the core members list. The list seems super big comparing to who is actually active on the project; in a meritocracy world it would make sense to revisit that list. Thanks, [1] https://review.openstack.org/#/admin/groups/124,members [2] http://stackalytics.com/report/contribution/packstack/90 -- Emilien Macchi