Continuation of the `docker secret` storage feature

Closes https://github.com/dotcloud/docker/pull/6075

Starting on a new PR to accommodate alex being on holiday 
it seems useful to me to add a local secret file mapped into my `/etc/apt/sources.d` so that I get my local debian repo - but doing so will either leave a dangling sofltlink, or some kind of confusion.

I'm starting to think that `secret` is a wrong name for this.