unbound(role): fix unbound prometheus exporter install on debian

- download debian image from github und verify checksum
- setup systemd service unit

roles/unbound/files/prometheus-unbound-exporter.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=Prometheus exporter for Unbound metrics, written in Go with pluggable metric collectors. The metrics exporter converts Unbound metric names to Prometheus metric names and labels by using a set of regular expressions.
+Documentation=https://github.com/letsencrypt/unbound_exporter
+After=network.target
+
+[Service]
+Type=simple
+EnvironmentFile=/etc/prometheus-unbound-exporter.conf
+ExecStart=/bin/unbound_exporter ${UNBOUND_EXPORTER_ARGS}
+
+[Install]
+WantedBy=multi-user.target