Support "mixed-mode" configuration

Previously, if `node["postgresql"]["conf"]` was set, a custom-only configuration file was used. Now, the default template will be used and will append any options in the `conf` hash, which makes it easier to extend the configuration without uncommenting all of the existing values. 

If `node["postgresql"]["conf_custom"]` is true, the default template will not be used and only values found in the `conf` hash will be written to the configuration.