Yeah, we're using configuration management, but that still seems too low level. I haven't tried CFEngine, but in the ones I have, there's no concept of an instance (essentially some configuration files and a few databases) that you should be able to treat as a single unit (e.g. delete instance or move it to another server).
I wonder what's the architecture behind WPEngine and similar services. It must provide some isolation since clients can install their own plugins, but on the other hand I don't see them creating a new Docker image for each client, especially since they're self-managed.
http://en.wikipedia.org/wiki/CFEngine
http://cfengine.com/product/community/