I had similar problems with rmilter on Ubuntu 14.04. It seems the installation of it from the rspamd repository is missing some config files. The fixes have been put in place, but there hasn't been a new release yet that includes them.
You can follow through the issue here:
https://github.com/vstakhov/rmilter/issues/66You'll also need to manually copy the rmilter.conf, rmilter.conf.common and rmilter.conf.sysvinit files to /etc/, you can get the files here:
https://github.com/vstakhov/rmilter/tree/master/debian