Move tox_envlist variable for releasenotes into job

The tox_envlist for the job that builds release note was
set in the post.yaml to fetch the tox output only.  However,
becasue it was not specified in the job, tox-siblings which
runs in the `pre` stage of `tox` would attempt to use `venv`
in the envlist field.

For jobs that do not have that venv (but do use release notes),
it would result in a failure.  This patch moves the variable out
into the job so that it's applied for the `tox-siblings` role.

Change-Id: If660822c763a973963be0c283dfdbebd97f06985

playbooks/releasenotes/post.yaml

@@ -1,7 +1,6 @@
 - hosts: all
   roles:
     - role: fetch-tox-output
-      tox_envlist: releasenotes
     - role: fetch-sphinx-output
       sphinx_output_src: "{{ zuul.project.src_dir }}/doc/build/html/"
       zuul_executor_dest: "{{ zuul.executor.log_root }}"

zuul.d/jobs.yaml

@@ -391,6 +391,8 @@
       - ^releasenotes/.*
       - ^tox.ini
       - ^.*requirements.txt$
+    vars:
+      tox_envlist: releasenotes
 
 - job:
     name: legacy-base