feat: Add systemd file and data to integrate the app as a daemon

data/etc/default/revpi-middleware

@@ -0,0 +1,8 @@
+# Additional options that are passed to revpi-middleware.
+#   add '-f /var/log/revpi-middleware.log' to write logs to own log file
+#   add '-v' or '-vv' for verbose logging
+
+DAEMON_OPTS=""
+
+# In addition to journalctl, use your own additional log file
+# DAEMON_OPTS="-f /var/log/revpi-middleware.log"