Add bindep.txt

The Vault functional tests are currently failing in the gate, and it
appears to be because the unzip binary is not present. We need to
add it to bindep so it will be installed before running tests.

Change-Id: I4b8075429218a494f75f5e3611f48927d18f1700
Closes-Bug: 1840061

bindep.txt

@@ -0,0 +1,4 @@
+# This is a cross-platform list tracking distribution packages needed for install and tests;
+# see https://docs.openstack.org/infra/bindep/ for additional information.
+
+unzip

tox.ini

@@ -104,3 +104,15 @@ deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
+
+[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