Add plan for kilo blueprints: when is a blueprint needed

At the nova mid-cycle we agreed that while specs have made our
process significantly better, we think the current approach still leaves
much to be desired. This document is the outcome of the discussion at
the mid-cycle on how we want to manage blueprints in kilo.

This proposal is being placed in devref, as we want to use gerrit's
workflow (two core reviews, inline comments, revisions etc) and this is
a document for developers.

It turns out adding the specs requirement to blueprints adds a
non-trivial amount of overhead, revisit when a blueprint and spec is
needed to minimize overhead in cases where it isn't helpful.

Change-Id: Iaaadc26f3f51ee091419beaa12e0508623eed99e
Merge "Add plan for kilo blueprints: when is a blueprint needed"