Replace openvpn-exporter with textfile exporter.

The upstream openvpn-exporter was emitting high cardinality metrics, because of our CN being an unique identifier based off of the certificate and the metrics are designed to emit individual metrics for each CN, even when you pass a flag that disables individual metrics. This would cause the tsdb to fill with unnecessary, and unrelated metrics.

This removes that exporter (which has not seen commits for years) and replaces it with the prometheus-node-exporter textfile collector, which will emit metrics produced by the shell script that is run every 10 seconds. It will only emit these metrics:

openvpn_server_connected_clients{status_path="/tmp/openvpn-status-tcp"}
openvpn_server_connected_clients{status_path="/tmp/openvpn-status-udp"}
openvpn_up{status_path="/tmp/openvpn-status-tcp"}
openvpn_up{status_path="/tmp/openvpn-status-udp"}