Need NRPE monitors for OVN status

[Xav Paice]

Following on from LP: #1896674, we should monitor for the following:

ovn-central:

Ussuri:
ovn-appctl -t /var/run/ovn/ovnnb_db.ctl cluster/status OVN_Northbound
ovn-appctl -t /var/run/ovn/ovnsb_db.ctl cluster/status OVN_Southbound

Train:
ovs-appctl -t /var/run/openvswitch/ovnsb_db.ctl cluster/status OVN_Southbound
ovs-appctl -t /var/run/openvswitch/ovnnb_db.ctl cluster/status OVN_Northbound

Check that:
Status: cluster member

ovn-chassis (and dedicated-chassis):
ovs-appctl connection-status (Needs more details)

Given that these commands require root, and that NRPE does not run as root, we will need the checks to be run via cron and results stored in a file, which NRPE checks can read. We will also want to check for the freshness of the file.

[Xav Paice (xavpaice)]

https://review.opendev.org/c/x/charm-ovn-central/+/758339

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/x/charm-ovn-central/+/819317

[OpenStack Infra (hudson-openstack)]

Change abandoned by "Przemysław Lal <email address hidden>" on branch: master
Review: https://review.opendev.org/c/x/charm-ovn-central/+/819317
Reason: The community won't accept any new nrpe checks.