caddy-frontend: Expose backend log files to slaves

Backend logs are exposed as usual access and error logs.

By using rsyslogd templates and regex filtering, the rsyslogd reacts itself
and creates needed files per each slave which accesses it. Thanks to this, it's
configuration is static from point of view of SlapOS profiles, and can be
generated once.

As the rsyslogd configuration became fully special to backend-haproxy, the
rsyslogd template filename and its references has been correctly renamed.