Add bindep target to tox

bindep is a helpful tox target to assist in determining
what components a test environment needs to have installed.

For stx-puppet, puppet-lint needs ruby headers otherwise
the tox linters target will fail.

Partial-Bug: #1907678
Signed-off-by: albailey <Al.Bailey@windriver.com>
Change-Id: Iaccd8d8f3af292ef29028cde59f0d344b94f1d72

bindep.txt

@@ -0,0 +1,2 @@
+ruby-devel [test platform:rpm]
+ruby-dev [test platform:dpkg]

tox.ini

@@ -33,6 +33,14 @@ commands =
          -i E006          \
          -e 'E*'"
 
+[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.
+deps = bindep
+commands = bindep test
+
 [testenv:linters]
 # Note: centos developer env requires ruby-devel
 # Ubuntu developer env requires ruby-dev