Create zuul_executor_dest variable for fetch-sphinx-output

It is possible we want to upload this data into a different directory
on the executor; expose that.

Change-Id: I377f80fb245beb5165a24205ed1bb954d4f8af42
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

roles/fetch-sphinx-output/defaults/main.yaml

@@ -1,2 +1,3 @@
 ---
+zuul_executor_dest: "{{ zuul.executor.log_root }}"
 zuul_work_dir: "src/{{ zuul.project.canonical_name }}"

roles/fetch-sphinx-output/tasks/main.yaml

@@ -1,6 +1,6 @@
 - name: Collect sphinx build html
   synchronize:
-    dest: "{{ zuul.executor.log_root }}"
+    dest: "{{ zuul_executor_dest }}"
     mode: pull
     src: "{{ zuul_work_dir }}/doc/build/html"
     verify_host: true