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/tox-post.yaml b/playbooks/tox-post.yaml
index 697b727..3b035f8 100644
--- a/playbooks/tox-post.yaml
+++ b/playbooks/tox-post.yaml
@@ -3,7 +3,7 @@
     - name: Find tox directories to synchrionize.
       find:
         file_type: directory
-        paths: "/home/zuul/workspace/src/{{ zuul.project }}/.tox"
+        paths: "{{ zuul_workspace_root }}/src/{{ zuul.project }}/.tox"
         # NOTE(pabelanger): The .tox/log folder is empty, ignore it.
         patterns: ^(?!log).*$
         use_regex: yes