Allow custom content in rsyslog.conf

[nick-markowski]

Rsyslog.conf is effectively templated to include a rule directory, and that's it. Update $_rsyslog_conf to include custom content or create a real template. Something like:

  $_rsyslog_conf = @("RSYSLOG_CONF"/$)
    # This file is managed by Puppet (simp/rsyslog module).
    # Any changes will be overwritten.
    \$IncludeConfig ${rsyslog::rule_dir}/*.conf
    ${rsyslog::custom_conf_content}
    | RSYSLOG_CONF