Use zuul-jobs style var names in run-docs

This job still needs to be updated to replace the jenkins script
with ansible.  But so that we can go ahead and remove the
zuul_workspace_root variable from everywhere, replace it with
the variables that we use for the tox jobs in zuul-jobs.

Change-Id: I991dc6c60fb7290be40e8d89ab2cdeb7ab8879c4

roles/run-docs/defaults/main.yaml

@@ -1,2 +1,3 @@
 ---
-envlist: venv
+tox_envlist: venv
+zuul_work_dir: "src/{{ zuul.project.canonical_name }}"

roles/run-docs/tasks/main.yaml

@@ -1,4 +1,4 @@
 - name: Execute run-docs.sh.
-  shell: "/usr/local/jenkins/slave_scripts/run-docs.sh {{ envlist }}"
+  shell: "/usr/local/jenkins/slave_scripts/run-docs.sh {{ tox_envlist }}"
   args:
-    chdir: "{{ zuul_workspace_root }}/src/{{ zuul.project.canonical_name }}"
+    chdir: "{{ zuul_work_dir }}"