Fix database configuration in heat.conf for YUM

Follow-up to https://review.openstack.org/#/c/61402
The Heat installation notes refer to the database configuration as done
in master, but it doesn't work completely with the new format in Havana.
Let's keep the old format for now.

Change-Id: I8f47206b0ae4856c67139c0b3634bb7880c37cc9
Closes-Bug: #1257436
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>