[Scalingo.json] Add variable we can use in template

[Frenchix62]

I see we can use for example this variable %APP_HOSTNAME% for just to retrieve the hostname of an app without http or https.
I also see this : "template": "%URL_WITHOUT_HTTPS%"

So is it possible to list on our API doc (https://developers.scalingo.com/scalingo-json-schema/index#the-manifest) on the template section, all the variables we can use. Actually there is just APP, PARENT_APP or PR_NUMBER