sysutils/loki: add promtail rc.d script

Promtail is a log reader and it can be run as a service to gather the
logs to be sent to remote loki server. For nodes that need to send logs
only, promtail service and configuration file is needed

PR: 274194, 274473
Signed-off-by: Christopher Beppler <freebsd@funzi.org>
Approved by: Chirstopher Beppler <freebsd@funzi.org> (maintainer)