Comment sample secrets.yml file

Created by: rotated8

(As from the install scripts:) Add comments to the secrets.yml.sample file that is intended to be edited and dropped into files/ during bootstrap. The comments give some guidance as to the meanings of the various YAML keys. Keys that are not normally present are included but commented out.

Also add some extra anchors to the default section so that individual items may be overridden, like that done for fedora:. These anchors are also included in the vanilla secrets.yml file generated if no files/secrets.yml file is present when running bootstrap.sh.