ansible-pull(role): export last successful run timestamp statistic

Using the node exporter textfile collector, expose the timestamp of the
last successful ansible-pull run.

roles/ansible_pull/templates/ansible-pull.service.j2

@@ -19,5 +19,6 @@ ExecStart=/usr/local/lib/ansible_pull_venv/bin/ansible-pull \
           --inventory "{{ ansible_pull__inventory }}" \
           "{{ ansible_pull__playbook }}"
 User={{ ansible_pull__user }}
+ExecStartPost=/usr/bin/bash -c 'echo "ansible_pull_last_successful_run_timestamp_seconds $(date +%s)" > /var/lib/node_exporter_textfile/ansible_pull.prom.$$; mv /var/lib/node_exporter_textfile/ansible_pull.prom.$$ /var/lib/node_exporter_textfile/ansible_pull.prom'
 # Reboot, if /var/run/reboot-required or /var/run/ansible-reboot-required exist.
 ExecStartPost=/usr/bin/bash -c 'if [ -e /var/run/reboot-required ] || [ -e /var/run/ansible-reboot-required ]; then sudo systemctl reboot; fi'