Update Ubuntu Vagrant bootstrap script

[Diogo Monteiro]

Currently the Vagrant ubuntu-bootstrap.sh script configures the docker daemon using the upstart approach that was the default on Trusty.
On Vivid Ubuntu uses systemd instead of upstart as the default boot manager so we need to configure the docker daemon using the systemd way.

Also the docker registry keeps reporting an IntegrityError everytime an image gets pushed: IntegrityError: (IntegrityError) UNIQUE constraint failed: repository.name u'INSERT INTO repository
The error can be tracked to this github issue: https://github.com/docker/docker-registry/issues/518
Based in one of the comments a possible reason could be:
"I expect the race is just a bunch of Gunicorn threads all trying to create the database at the same time. Here's how the initial index generation is supposed to work [1], and here's using --preload to avoid the race [2,3,4]. "

After passing the --preload flag when creating the registry the IntegrityError was gone.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/251594
Committed: https://git.openstack.org/cgit/openstack/kolla/commit/?id=c7297e04c15eb3dd7fbf0825c51932452c8c1275
Submitter: Jenkins
Branch: master

commit c7297e04c15eb3dd7fbf0825c51932452c8c1275
Author: Diogo Monteiro <email address hidden>
Date: Mon Nov 30 19:23:16 2015 -0500

    Update Vagrant's Ubuntu box provisioning script

    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