issues relating to nagios and lack of nrpe on nagios unit

Stable charms
Bionic

When relating rsyslog-forwarder-ha to the nagios unit, an error appears:

---

2020-08-05 13:23:57 ERROR juju.worker.uniter.operation runhook.go:132 hook "config-changed" failed: exit status 1
2020-08-05 13:25:19 DEBUG config-changed Traceback (most recent call last):
2020-08-05 13:25:19 DEBUG config-changed File "/var/lib/juju/agents/unit-rsyslog-forwarder-ha-1/charm/hooks/config-changed", line 294, in <module>
2020-08-05 13:25:19 DEBUG config-changed hooks.execute(sys.argv)
2020-08-05 13:25:19 DEBUG config-changed File "/var/lib/juju/agents/unit-rsyslog-forwarder-ha-1/charm/hooks/charmhelpers/core/hookenv.py", line 909, in execute
2020-08-05 13:25:19 DEBUG config-changed self._hooks[hook_name]()
2020-08-05 13:25:19 DEBUG config-changed File "/var/lib/juju/agents/unit-rsyslog-forwarder-ha-1/charm/hooks/config-changed", line 287, in config_changed
2020-08-05 13:25:19 DEBUG config-changed update_nrpe_config()
2020-08-05 13:25:19 DEBUG config-changed File "/var/lib/juju/agents/unit-rsyslog-forwarder-ha-1/charm/hooks/config-changed", line 123, in update_nrpe_config
2020-08-05 13:25:19 DEBUG config-changed nrpe_compat.write()
2020-08-05 13:25:19 DEBUG config-changed File "/var/lib/juju/agents/unit-rsyslog-forwarder-ha-1/charm/hooks/charmhelpers/contrib/charmsupport/nrpe.py", line 301, in write
2020-08-05 13:25:19 DEBUG config-changed self.nagios_servicegroups)
2020-08-05 13:25:19 DEBUG config-changed File "/var/lib/juju/agents/unit-rsyslog-forwarder-ha-1/charm/hooks/charmhelpers/contrib/charmsupport/nrpe.py", line 195, in write
2020-08-05 13:25:19 DEBUG config-changed with open(nrpe_check_file, 'w') as nrpe_check_config:
2020-08-05 13:25:19 DEBUG config-changed IOError: [Errno 2] No such file or directory: '/etc/nagios/nrpe.d/check_rsyslog.cfg'

---

It appears the r-f-ha is expecting nrpe to be here. from the code:

---
@hooks.hook("nrpe-external-master-relation-changed")
@hooks.hook("local-monitors-relation-changed")
def update_nrpe_config():
    nrpe_compat = nrpe.NRPE()
    nrpe_compat.add_check(
        shortname="rsyslog",
        description="Check rsyslog is running",
        check_cmd="check_procs -c 1: -C rsyslogd"
    )
    nrpe_compat.write()

---

This works on all other units since we have nrpe-{host|container} there, but it is redundant and not sensible to have nrpe on a nagios unit.

running 'mkdir -p /etc/nagios/nrpe.d' on the nagios unit where r-f-ha is at resolves the issue.