Install graphviz for building docs

[Matt Riedemann]

Nova bug 1356983 was to update the documentation in nova to install graphviz from the distro so you can run 'tox -e docs' with sphinx.

We should install this package in devstack via the distro also, it's the same name regardless of Ubuntu vs RHEL/Fedora.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/115989

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/115989
Committed: https://git.openstack.org/cgit/openstack-dev/devstack/commit/?id=18d5db8452fb8fc5bfb55c4e9c7aa5f03cdbe829
Submitter: Jenkins
Branch: master

commit 18d5db8452fb8fc5bfb55c4e9c7aa5f03cdbe829
Author: Matt Riedemann <email address hidden>
Date: Wed Aug 20 18:09:44 2014 -0700

    Add graphviz package to general list for building docs

    A few projects are now gating on docs not having warnings/errors in them
    during the docs build with sphinx, so developers are going to want to be
    able to run 'tox -e docs' locally and fix issues.

    The graphviz package is used by sphinx when building docs but you have
    to get it from the distro, so this change adds the package to the
    various distro lists.

    Closes-Bug: #1359749

    Change-Id: I7b7dee5e5d0dce6d5020b3e711f500118be163f8