Allow docker to free container labels when containers are removed.

Currently we do not remove reservations on MCS labels when a container
is removed.  Not a big problem, since on reboot it would be freed.
But we should be cleaning this up.  Currently we support ~500,000
labels.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Merge pull request #162 from rhatdan/unreserve

Allow docker to free container labels when containers are removed.