Fixed AMQP configuration steps for Block Storage

I fixed the AMQP configuration steps in cinder.conf for Block Storage
on both controller and service nodes. I also unified structure and
phrasing with other portions of the guide and mentioned replacing
'RABBIT_PASS'. This patch affects all distributions.

I also found and fixed a minor issue causing several database
configuration instructions for other distributions to leak into those
for Debian which uses Debconf.

Change-Id: Ifd6f185b422195a00aad6efa83418d34b93eac85
backport: havana
Closes-Bug: #1244781
(cherry picked from commit a5fd95d740fa0e6c992dfdd401e2e64d050e5600)