Switch releasenotes to fetch-sphinx-tarball

As step to use promote job for publishing releasenotes, we need to use
fetch-sphinx-tarball for this job.

Update success-url to docs to match the role.

Change-Id: I5aabf5cfadee1e12baab117a20b1f50766f3d7fc
Needed-By: https://review.opendev.org/678430

playbooks/releasenotes/post.yaml

@@ -1,4 +1,4 @@
 - hosts: all
   roles:
-    - role: fetch-sphinx-output
-      sphinx_output_src: "{{ zuul_work_dir|default(zuul.project.src_dir) }}/releasenotes/build/html"
+    - role: fetch-sphinx-tarball
+      sphinx_build_dir: "releasenotes/build"

zuul.d/python-jobs.yaml

@@ -307,7 +307,7 @@
          :default: {{ zuul.project.src_dir }}
 
          Directory to operate in.
-    success-url: html/
+    success-url: docs/
     # Release notes always build on master.
     override-checkout: master
     pre-run: playbooks/releasenotes/pre.yaml