persist memcached logs in /var/log/containers/memcached/memcached.log
persist memcached logs in /var/log/containers/memcached/memcached.log

We used to bind-mount /var/log/memcached.log, but this resulted in the
file being createdin the memcached container as a directory, since this
file didn't exist.

This commit takes the approach of other containers and gets the logs to
a memcached directory in /var/log/containers.

Change-Id: I926b65fa557ad56b4faa2be34452b58f7b01247a
Closes-Bug: #1720183

openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates