This systemd.exec setting will construct a new mount namespace for the
docker daemon, and use slave shared-subtree mounts so that volume mounts
propogate correctly into containers.

By having an unshared mount namespace for the daemon it ensures that
mount references are not held by other pids outside of the docker
daemon. Frequently this can be seen in EBUSY or "device or resource
busy" errors.

Signed-off-by: Vincent Batts <vbatts@redhat.com>