Fix outdated DB connection information

In Icehouse, the format of heat.conf has changed to be more consistent
with other projects.  What used to be:
[DEFAULT]
sql_connection= ...

Is now:

[database]
connection= ...

This patch fixes the heat module so that it correctly populates
database connection information.

Change-Id: I6edc9c7b58dcd5569f20f202b9b665431c7b0845
Closes-Bug: #1308868