From what I could gather during the introductory webinar sgordon's answer is correct.
It must be noted that whilst MCP retains a component called MOS, it's a completely different beast from the old MOS that used to be provisioned by Fuel/Puppet. I attended Mirantis' OpenStack BCN presentation on the architecture, which also featured a demo, useful for grasping how different - and indeed more advanced/complex - the MCP-provisioned MOS is.
My gut feel: MCP is a giant step in the direction of a much more supportable architecture (I happen to like Salt too, which is used instead of Puppet for provisioning) at the expense of a big uptake in complexity. Having a platform that is indeed upgradable is probably worth a few sacrifices on that front...
There are two important corollaries as far as I am concerned:
- Old Puppet/Fuel based MOS is being EOLed (though not before 2019 and possibly longer if you have a support contract)
- Unlike MOS, you cannot just download MCP and take it from a spin - with a view of perhaps enrolling into a support contract with Mirantis once your setup reaches critical mass. The consumption model involves upfront contracting and a minimal period of hand-holding, during which Mirantis mostly operate your platform for you; likely due to aforementioned complexity.
(Sorry for separate answer due to comment size limit)