[qa] devstack & hacking core member updates

Hello everyone, I would like to propose the following updates to the devstack and hacking project core member list: Devstack: - Adding gibi in devstack core. He has been fixing and helping in devstack for a long time. Hacking: - Adding stephenfin and sean-k-mooney in the hacking core members list. Both have a good understanding of hacking projects and helping there on a need basis. - Frickler and I are part of the hacking core, but via included groups. I will be moving both to the core member list explicitly. Feel free to provide your feedback. If there are no objections by this Friday (21st Nov), I will update the gerrit group. -gmaan