Add plan for kilo blueprints: project priorities

At the nova mid-cycle we agreed that while specs have made our blueprint
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.

Introduce the idea of project priorities.

Change-Id: I7075a130c01be22f3ee5315d4a65b8e524b5a1ad
Merge "Add plan for kilo blueprints: project priorities"