Pushing out validation to the top level MutiConfig

We have partial blocks at all other stages. I've decoupled the
validation from the raw JSON loader, so that we can ensure the *final*
object is sane, not every object along the way.

Many thanks to Phillip Susi, who's upload caused this bug.

Signed-off-by: Paul Tagliamonte <paultag@debian.org>
Fixing a typo on the last commit