Add the start of a Zuul rolling restart playbook

This handles rolling the mergers and executors, but not yet
the schedulers.

Also, it does the executors in complete batches of 6, but could be
improved to stop 6 and then do each of the next as the first ones
complete.

Change-Id: I2dca104194c2f129b68dcef7721d7d08cb987c46

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

@@ -0,0 +1,18 @@
+- name: Gracefully stop Zuul Executor
+  shell:
+    cmd: docker-compose exec executor zuul-executor graceful
+    chdir: /etc/zuul-executor
+  become: true
+  become_user: root
+- name: Wait for Zuul Executor to stop
+  shell:
+    cmd: docker-compose ps -q | xargs docker wait
+    chdir: /etc/zuul-executor
+  become: true
+  become_user: root
+- name: Remove Zuul Executor containers
+  shell:
+    cmd: docker-compose down
+    chdir: /etc/zuul-executor
+  become: true
+  become_user: root

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

@@ -0,0 +1,18 @@
+- name: Gracefully stop Zuul Merger
+  shell:
+    cmd: docker-compose exec merger zuul-merger stop
+    chdir: /etc/zuul-merger
+  become: true
+  become_user: root
+- name: Wait for Zuul Merger to stop
+  shell:
+    cmd: docker-compose ps -q | xargs docker wait
+    chdir: /etc/zuul-merger
+  become: true
+  become_user: root
+- name: Down Zuul Merger containers
+  shell:
+    cmd: docker-compose down
+    chdir: /etc/zuul-merger
+  become: true
+  become_user: root

playbooks/zuul_rolling_restart.yaml

@@ -0,0 +1,23 @@
+# Run this with "-f 20"
+
+- hosts: 'zuul-merger:!disabled'
+  tasks:
+    - include_role:
+        name: zuul-merger
+        tasks_from: graceful
+
+- hosts: 'zuul-executor:!disabled'
+  serial: 6
+  tasks:
+    - include_role:
+        name: zuul-executor
+        tasks_from: graceful
+    - include_role:
+        name: zuul-executor
+        tasks_from: start
+
+- hosts: 'zuul-merger:!disabled'
+  tasks:
+    - include_role:
+        name: zuul-merger
+        tasks_from: start