Github chef/client-core and chef/client-maintainers groups


Just a quick note that I eliminated the old client-engineers github
group which was pretty stale this morning (and reflected an
internal/external maintainer split that I believe we want to be
avoiding). Replaced that with the client-maintainers group. The
client-core group is now faithful to the MAINTAINERS doc for the core
maintainers, and the client-maintainers groups is similarly faithful to
all the users listed in the doc.