Provide an additional_params section to nova.conf

In order to support obscure nova.conf parameters (there are lots of
them), add an additional_params section such that a user can provide
a hash of key value pairs. Not only does this ungate users from making
the most use of these cookbooks, but also future proofs them a bit.

Change-Id: I181874d866df02660b7edb9d70611fb0f0c8ada0