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
18d5db8452