Need to document installing graphviz from distro for tox -e docs run

[Matt Riedemann]

This commit:

https://github.com/openstack/nova/commit/a507d42cf5d9912c2b3622e84afb8b7d3278595b

Made warnings get treated like errors when building docs, therefore, people are going to need to get used to running 'tox -e docs' locally. However, you need the graphviz package for that to work which comes from the distro, so we need to document that in the developement environment setup doc here:

http://docs.openstack.org/developer/nova/devref/development.environment.html#linux-systems

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/114301

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/114301
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=6b796597d57ebbd1b0a0c31195dd7b5aa53fce7a
Submitter: Jenkins
Branch: master

commit 6b796597d57ebbd1b0a0c31195dd7b5aa53fce7a
Author: Matt Riedemann <email address hidden>
Date: Thu Aug 14 10:27:28 2014 -0700

    Add graphviz to list of distro packages to install

    Commit a507d42cf5d9912c2b3622e84afb8b7d3278595b makes the
    doc builds fail on warnings, so people are probably going
    to be running 'tox -e docs' more often. To run that you
    need the graphviz package from the distro, so add it to
    the list of packages needed when setting up a development
    environment.

    Change-Id: I47ffe63fddfbbd73c3fc698490ad07235caaf459
    Closes-Bug: #1356983