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 
This example does not show how one would use the secrets file, nor how to clean up afterwards - for example - are you expecting users to softlink the secret file to its expected location?

How do you then manage the hidden dependency on a file that is then gone? will a subsequent `docker run` give the user a sensible error message (before the container starts?)