Fix docker wait requires at least one argument

In the graceful shutdown for mergers and executors if we skip the docker
exec to stop the container we also need to skip the docker wait. The
reason for this is docker wait exits with an error code if not provided
with any arguments to wait for.

Change-Id: Id09666ee23e1a9599d477b63a89559e4ab1d21bf

playbooks/roles/zuul-executor/tasks/graceful.yaml

@@ -20,6 +20,7 @@
     chdir: /etc/zuul-executor
   become: true
   become_user: root
+  when: executor_container_list.stdout_lines | length > 0
 - name: Remove Zuul Executor containers
   shell:
     cmd: docker-compose down

playbooks/roles/zuul-merger/tasks/graceful.yaml

@@ -20,6 +20,7 @@
     chdir: /etc/zuul-merger
   become: true
   become_user: root
+  when: merger_container_list.stdout_lines | length > 0
 - name: Down Zuul Merger containers
   shell:
     cmd: docker-compose down