List system dependencies for running common tests

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

tox.ini

@@ -131,3 +131,14 @@ import_exceptions = nova.i18n
 # of the requirements.txt files
 deps = pip_missing_reqs
 commands=pip-missing-reqs -d --ignore-file=nova/tests/* --ignore-file=nova/test.py nova
+
+[testenv:bindep]
+# 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