Add a bindep.txt file containing a cross-platform list
of dependencies needed for running included tox-based tests. Also
include a tox environment for convenience calling the bindep[*]
utility to list any missing system requirements, and simplify the
contributor documentation to recommend this rather than embedding an
ad-hoc list of package names in an untestable document.
This change is self-testing.
[*] http://docs.openstack.org/infra/bindep/
Change-Id: I7c18b0921c39b99df89f2c356208303c2bad5a97
Jeremy Stanley