Add missing host_prep_tasks for memcached

I missed this in change
Iecce2820657c5ad6bd2b02849c0433e5dce8009b. Memcached seems to be the
only service which mounts the /var/log/container/<service-name>
directory directly under /var/log in the container.

Change-Id: I26928bc7ad5f2f1bed60d200e8f2f0c310a14ee5
Partial-Bug: #1730957

docker/services/memcached.yaml

@@ -103,6 +103,11 @@ outputs:
             # module, but we'll only be able to do this once the following pull
             # request merges: https://github.com/saz/puppet-memcached/pull/88
             command: ['/bin/bash', '-c', 'source /etc/sysconfig/memcached; /usr/bin/memcached -p ${PORT} -u ${USER} -m ${CACHESIZE} -c ${MAXCONN} $OPTIONS >> /var/log/memcached.log 2>&1']
+      host_prep_tasks:
+        - name: create persistent logs directory
+          file:
+            path: /var/log/containers/memcached
+            state: directory
       upgrade_tasks:
         - name: Stop and disable memcached service
           tags: step2