Fix matrix-prometheus-node-exporter failure to start

The quotes around "host" for both `--pid` and `--net` were
causing trouble for me:

> docker: --pid: invalid PID mode.

and:

> docker: Error response from daemon: network "host" not found.

I've also changed the `-v` call to `--mount` for consistency with the
rest of the playbook.

roles/matrix-prometheus-node-exporter/templates/systemd/matrix-prometheus-node-exporter.service.j2

@@ -27,9 +27,9 @@ ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-prometheus-nod
 			{% for arg in matrix_prometheus_node_exporter_container_extra_arguments %}
 			{{ arg }} \
 			{% endfor %}
-			--net="host" \
-			--pid="host" \
-			-v "/:/host:ro,rslave" \
+			--net=host \
+			--pid=host \
+			--mount type=bind,src=/,dst=/host,ro,bind-propagation=rslave \
 			{{ matrix_prometheus_node_exporter_docker_image }} \
 			--path.rootfs=/host