Add local bindep.txt

As it was announced [1] global bindep-fallback.txt was removed and now
projects need to have a local bindep.txt to be able to install binary
dependencies for testing.

In documentation zuul job graphviz package ('dot' command) is needed.
Without that the job fails with:
    dot command 'dot' cannot be run (needed for graphviz output),
    check the graphviz_dot setting

[1] http://lists.openstack.org/pipermail/openstack-discuss/2019-June/007272.html

Change-Id: I5efddfc72e9dc0dd1b8b9e1e37708e1d30ef3c39

bindep.txt

@@ -0,0 +1,7 @@
+# This is a cross-platform list tracking distribution packages needed for install and tests;
+# see https://docs.openstack.org/infra/bindep/ for additional information.
+
+# graphviz is necessary for documentation build
+graphviz [!platform:gentoo]
+media-gfx/graphviz [platform:gentoo]
+

tox.ini

@@ -48,6 +48,18 @@ deps =
   -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
+[testenv:bindep]
+basepython = python3
+# Do not install any requirements. We want this to be fast and work even if
+# system dependencies are missing, since it's used to tell you what system
+# dependencies are missing! This also means that bindep must be installed
+# separately, outside of the requirements files, and develop mode disabled
+# explicitly to avoid unnecessarily installing the checked-out repo too (this
+# further relies on "tox.skipsdist = True" above).
+deps = bindep
+commands = bindep test
+usedevelop = False
+
 [testenv:lower-constraints]
 basepython = python3
 deps =