Rename secrets.yml to secrets.prod.yml.
Ignore the One True Secrets File.
Let's call the blank config "nodeless".
Now with a correct comment.
Default secrets appropriately.

`vagrant up`, via `dev_web.yml`, defaults to `secrets.nodeless.yml`.
`script/deploy`, via `site.yml`, defaults to `secrets.prod.yml` and needs
the vault password.
Fixes my syntax highlighting.