Starting from Vivid Ubuntu uses systemd as the
default boot manager instead of upstart. Updating
the way the docker daemon is configured to follow
the systemd way.
Setting env variable GUNICORN_OPTS=[--preload]
to fix race condition and avoid IntegrityError
when pushing images to local registry

Change-Id: I841f85015c2c3683ef7e1e454ec1369263063c45
Related-bug: https://github.com/docker/docker-registry/issues/518
Closes-Bug: #1523336