Remove write_only_yaml behavior from catalog preview

The write_only_yaml terminus is going away in puppet 6, see PUP-8528. So catalog preview needs to be updated to work with it (since trying to query a non-existent setting will raise an error). Related question, I don't understand why catalog preview defaults the fact terminus to `yaml`, because it's unlikely that the facts for an agent are located there. Using puppetdb is better, but it seems like the best thing would be for catalog preview to take a facts file for the node you're previewing catalogs for, which is likely different the puppetserver host on which you're running catalog preview.