Amended config instructions for cinder.config

Changed instructions from editing the [database] section to editing the
[DEFAULT] section, and slightly amended parameter.

Change-Id: I3cb6a59f7ea41486dbdf66611797f58eea3be1a3
backport: havana
Closes-Bug: #1245880
(cherry picked from commit adfc7259f42f80588e138a3c210be0b53a89af8b)