Add MAINTAINERS.rst

Add a MAINTAINERS.rst file with two goals

1) to help seed initial reviews with relevant people.  Devstack by its
nature attracts new contributors, so when they hit some obscure bug,
this will help them find out who to put on a review.

2) core reviewers can't know everything about everything.  Having
someone who has flagged themselves as interested in an area sign-off
off on relevant changes can help move changes through.

Change-Id: Idf0914f55d72832a829ac99edf763ff51f71ca6c
Merge "Add MAINTAINERS.rst"