Replace openvpn-exporter with textfile exporter.

The upstream openvpn-exporter was emitting high cardinality metrics, because of
the CN being an unique identifier based off of the certificate. 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"}