machine-id: add container id access

Something like access to a UUID, for containers.
https://github.com/dotcloud/docker/issues/3506

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
container id: more container identification info

* /etc/machine-id has downstream requirements, accomodate those.
* provide access to the container id _and_ name

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)