Start zuul-executor after afsd and /afs is mounted

Add an override to the systemd configuration for the docker service
unit so that it won't start until after openafs-client is started
and /afs is mounted (the latter because we don't know if the
initscript will possibly return early). Without this, it's a race to
see whether the container will have a working /afs mount, so can
lead to jobs failing to write into AFS with cryptic permissions
errors.

Change-Id: Ie00b1c1bc9c330e2af28c59b3b07a7c244c912dc

playbooks/roles/zuul-executor/files/after-afs.conf

@@ -0,0 +1,2 @@
+[Unit]
+After=afs.mount openafs-client.service

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

@@ -58,6 +58,16 @@
   vars:
     logrotate_file_name: /var/log/zuul/executor-debug.log
 
+- name: Make docker systemd override directory
+  file:
+    state: directory
+    path: /etc/systemd/system/docker.service.d
+
+- name: Install docker systemd override for openafs ordering
+  copy:
+    src: after-afs.conf
+    dest: /etc/systemd/system/docker.service.d/after-afs.conf
+
 - name: Make docker-compose directory
   file:
     state: directory