docker-admin is now a centos 7 container

The admin node has full functionality, but no care has been
taken to preserve backwards compatibility with a centos 6 based
admin node.

Part of that change was ditching using Apache for the provisioner
webserver in favor of a tiny static webserver in Go.
Auf wiedersehn, chef-server.  Hello, goiardi.

Smaller, less resource intensive, just about as fast.
What's not to like?