Add Secret store

This is a continuation of https://github.com/dotcloud/docker/pull/5836, and the discussions about it we had on last weeks irc meeting. It implements a secret store that you can access via:
* docker secret list
* docker secret add a-name a-file
* docker secret rm a-name

You can then grant access to a specific secret by using `docker run --grant-secret a-name image`.
The granted secrets are stored (by name) in the hostconfig, and will be re-granted on restarting the container (assuming the secret is still in the store). Additionally any files stored in /etc/docker/secrets are considered "host based" secrets and will be granted to all containers on the host.

On container start the secrets are copied into a tmpfs in /run/secrets, so they will not ever be part of any image based on the container.

There are no docs yet, I'll be working on that next.