Assign a topology spread constraint to the roundcube pods

When operating against an imap-murder and mariadb-galera cluster,
it is important that a user session stick's to a particular frontend.
To avoid having to assign specific egress ip's to the individual pods,
we spread the pods over all available workers, thus making sure we have
unique, per pod source ip's, with the caveat that we can only run as
many pods as we have workers.