refactor(scheduler): use coreos/fleet for container scheduling

This is a large PR that reflects wholesale changes to the way Deis manages containers.  Brace yourselves!

As a byproduct of implementing a proper scheduler abstraction, the host/node management features in Deis are no longer relevant and have been removed.  This dramatically simplifies the Deis codebase, since Deis does not have to be made aware of changes in host/node topology.  The hard Chef dependency has also been removed.

Given the disruptive nature of this PR, it will require significant changes to documentation which are a work-in-progress.