Be able to update workers.

Before this change, you couldn't actually update workers because their config never got updated and they never got restarted when you updated the edx-platform.  This will allow us to quickly update the workers.  We can't do this as part of the edxapp.yml play because that is used in multiple places.

playbooks/roles/edx_ansible/templates/update.j2

@@ -46,6 +46,7 @@ declare -A repos_to_cmd
 edx_ansible_cmd="{{ edx_ansible_venv_bin }}/ansible-playbook -i localhost, -c local $extra_args"
 
 repos_to_cmd["edx-platform"]="$edx_ansible_cmd edxapp.yml -e 'edx_platform_version=$2'"
+repos_to_cmd["edx-workers"]="$edx_ansible_cmd edxapp.yml -e 'edx_platform_version=$2' -e 'celery_worker=true'"
 repos_to_cmd["xqueue"]="$edx_ansible_cmd xqueue.yml -e 'xqueue_version=$2' -e 'elb_pre_post=false'"
 repos_to_cmd["cs_comments_service"]="$edx_ansible_cmd forum.yml -e 'forum_version=$2'"
 repos_to_cmd["xserver"]="$edx_ansible_cmd xserver.yml -e 'xserver_version=$2'"