Handle no running containers during zuul graceful stop

The way the currently graceful stop tasks are written for zuul expects
there to always be a running zuul container to exec into. There are
situations where there may not be a running container in which case
there is nothing to stop. Avoid this being an error by checking if the
containers are running before execing into them. If no containers are
running then we'll noop the docker exec step allowing the rest of the
ansible tasks to continue.

Change-Id: I6c47147a589ae12cc33e37e40e49673396d120f7

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

@@ -1,9 +1,19 @@
+- name: Check if Zuul Executor containers are running
+  # It is possible they are stopped due to some external circumstance
+  command:
+    cmd: docker-compose ps -q
+    chdir: /etc/zuul-executor
+  become: true
+  become_user: root
+  register: executor_container_list
 - name: Gracefully stop Zuul Executor
   shell:
     cmd: docker-compose exec executor zuul-executor graceful
     chdir: /etc/zuul-executor
   become: true
   become_user: root
+  # Only run the docker exec command if a container is running
+  when: executor_container_list.stdout_lines | length > 0
 - name: Wait for Zuul Executor to stop
   shell:
     cmd: docker-compose ps -q | xargs docker wait

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

@@ -1,9 +1,19 @@
+- name: Check if Zuul merger containers are running
+  # It is possible they are stopped due to some external circumstance
+  command:
+    cmd: docker-compose ps -q
+    chdir: /etc/zuul-merger
+  become: true
+  become_user: root
+  register: merger_container_list
 - name: Gracefully stop Zuul Merger
   shell:
     cmd: docker-compose exec merger zuul-merger stop
     chdir: /etc/zuul-merger
   become: true
   become_user: root
+  # Only run the docker exec command if a container is running
+  when: merger_container_list.stdout_lines | length > 0
 - name: Wait for Zuul Merger to stop
   shell:
     cmd: docker-compose ps -q | xargs docker wait