Add --grant-secret hostconfig option

If you run something like
  docker run --grant-secret foo image

Then the "foo" secret will be copied into the /run/secrets tmpfs in the
container. Additionally, all host-based secrets will be copied into
all containers running on a specific host.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)