Make sure releasenotes always build on master

reno expects to be run on the master branch, so add an 'override-branch'
setting to the job.

While we're in there, add a description and remove the tox_envlist var,
as it is hard-coded inside of the role.

Change-Id: Ide918d1bb631590683f96bddc6b43c12c7ab5ac2

roles/build-releasenotes/tasks/main.yaml

@@ -1,9 +1,3 @@
-# reno always runs on master, even if there was a commit from another branch
-- name: Switch to master branch
-  command: git checkout master
-  args:
-    chdir: '{{ zuul_work_dir }}'
-
 # TODO(mordred) Put the translations logic into a sphinx plugin?
 - name: Check if translations exist for release notes
   stat:

zuul.d/jobs.yaml

@@ -355,16 +355,18 @@
 - job:
     name: build-openstack-releasenotes
     parent: openstack-tox
+    description: |
+      Build releasenotes, with optional translation support, using reno.
     success-url: html/
     run: playbooks/releasenotes/run
     post-run: playbooks/releasenotes/post
+    # Release notes always build on master.
+    override-branch: master
     timeout: 1800
     files:
       - ^releasenotes/.*
       - ^tox.ini
       - ^.*requirements.txt$
-    vars:
-      tox_envlist: releasenotes
 
 - job:
     name: legacy-base