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/prepare-workspace/tasks/main.yaml b/playbooks/roles/prepare-workspace/tasks/main.yaml
index d8a5316..c5952c7 100644
--- a/playbooks/roles/prepare-workspace/tasks/main.yaml
+++ b/playbooks/roles/prepare-workspace/tasks/main.yaml
@@ -10,12 +10,12 @@
 
 - name: Create workspace directory.
   file:
-    path: "{{ prepare_workspace_root }}"
+    path: "{{ zuul_workspace_root }}"
     owner: zuul
     group: zuul
     state: directory
 
 - name: Synchronize src repos to workspace directory.
   synchronize:
-    dest: "{{ prepare_workspace_root }}"
+    dest: "{{ zuul_workspace_root }}"
     src: "{{ zuul.executor.src_root }}"