Migration to heat.conf breaks per-service config

It seems that the migration to a common heat.conf config file has been left incomplete, and in particular we can't deprecate or remove the per-service files until we figure out a way to e.g specify bind_port per service in the common file.

I guess this means we need to create a per-service (e.g heat-api, heat-api-cfn) subsection in the sample config, defaulted to the values in the current config files.

I'm not that familiar with oslo.cfg, but it would be very much preferred if we can do this in a backwards compatible way (ie so people can transition from the old per-service files to the consolidated file easily, which appears to be the case with the majority of other options.