tox: Speed up bindep target

There is no need to install tempest itself for this target.

Change-Id: I19c9898d1e2f88b46615b2a57def3e778228f73f
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
diff --git a/tox.ini b/tox.ini
index 5a06912..634b72f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -475,8 +475,8 @@
     pip-extra-reqs -d --ignore-file=tempest/tests/* tempest
     pip-missing-reqs -d --ignore-file=tempest/tests/* tempest
 
-
 [testenv:bindep]
+skip_install = true
 # 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