Enabling dynamic config files

[izaaklauer]

Prior to this, it appears that dynamic config targeting files did not work - our logic was treating all dynamic config as env vars, and not respecting name_is_path .

This enables stanzas like this to work:

 config {
   file = {
    "/shared-config/nginx.conf" = configdynamic("vault", {
    path = "secret/data/apps/sample-app"
     key = "data/nginx.conf"
    })
   }
 }

I can't claim to have deeply understood all of the surrounding logic, so if there's a better place or way to do this, I'm open to feedback.

This also incidentally adds some better error messages to the vault config-sourcer plugin.

[mitchellh]

Only comment I’d have is that internal/appconfig is built to be relatively easy (or at least, possible) to test. It’s one of the design goals of the package and separating it from plugins and so on so we can just simulate any behavior. I’d recommend getting a failing test in here.