This adds support for reloading the docker daemon
(SIGHIUP) so that changes in '/etc/docker/daemon.json'
can be loaded at runtime by reloading the service
through systemd ('systemctl reload docker')

Before this change, systemd would output an error
that "reloading" is not supported for the docker
service;

systemctl reload docker
Failed to reload docker.service: Job type reload is not applicable for unit docker.service.

After this change, the docker daemon can be reloaded
through 'systemctl reload docker', which reloads
the configuration;

journalctl -f -u docker.service

May 02 03:49:20 testing systemd[1]: Reloading Docker Application Container Engine.
May 02 03:49:20 testing docker[28496]: time="2016-05-02T03:49:20.143964103-04:00" level=info msg="Got signal to reload configuration, reloading from: /etc/docker/daemon.json"
May 02 03:49:20 testing systemd[1]: Reloaded Docker Application Container Engine.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>