Create zuul_workspace_root job variable

This will be a top-level variable used to define the workspace root
for our zuul user.

Change-Id: I4ecd6ead4975691798404f8d9510ed47e20e0c0c
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/playbooks/roles/run-tox/tasks/main.yaml b/playbooks/roles/run-tox/tasks/main.yaml
index 1053690..29a4cc4 100644
--- a/playbooks/roles/run-tox/tasks/main.yaml
+++ b/playbooks/roles/run-tox/tasks/main.yaml
@@ -1,4 +1,4 @@
 - name: Run tox
   shell: "/usr/local/jenkins/slave_scripts/run-tox.sh {{ run_tox_envlist }}"
   args:
-    chdir: "/home/zuul/workspace/src/{{ zuul.project }}"
+    chdir: "{{ zuul_workspace_root }}/src/{{ zuul.project }}"