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 
can you explain why these secrets are not as secret as the other secrets and don't need granting too?

I can see me having many distribution specific files in here, and wholesale tossing my apt cfg's into my centos containers seems bizzare.

also - rather than granting based on location on disk, why not persist the '*' setting for each named secret - and allowing it to be `docker secret set-access`

it also seems a shame that a secret 'name' can't be given to a set of dirs and files.

(I'm assuming that `/etc/docker/secrets/dir/dir2/files` will work)