Add the tox_install_bindep option to the tox job

If no binary deps are required by a tox run then
give the ability to not run the bindep role.

default is: true

Change-Id: Iacdf54e74126305199c17a932c001bc12c484759

playbooks/unittests/pre.yaml

@@ -1,5 +1,11 @@
 - hosts: all
-  roles:
-    - role: bindep
-      bindep_dir: "{{ zuul_work_dir }}"
-    - test-setup
+  tasks:
+    - name: Install binary dependencies
+      include_role:
+        name: bindep
+      vars:
+        bindep_dir: "{{ zuul_work_dir }}"
+      when: tox_install_bindep | default(true)
+    - name: Run test-setup role
+      include_role:
+        name: test-setup

zuul.yaml

@@ -84,6 +84,12 @@
 
          Override tox requirements that have corresponding zuul git repos
          on the node by installing the git versions into the tox virtualenv.
+
+      .. zuul:jobvar: tox_install_bindep
+         :default: true
+
+         Whether or not to run the binary dependencies detection and
+         installation with bindep.
     run: playbooks/tox/run.yaml
     pre-run: playbooks/tox/pre.yaml
     post-run: playbooks/tox/post.yaml