Openstack Integrator should have nrpe checks that monitor status of openstack components supporting k8s workloads

I think the best approach to fix this bug would be to add `layer:nagios` and create a python script in the templates folder. Each py file will contain one of the checks.

Here I provide my idea for nrpe check for all OpenStack networks should look like.
When the `nrpe-external-master.available` flag exists, the `check_openstack_networks.py` file will be installed as a nagios plugin. This file checks all OpenStack networks to see if they are in the ACTIVE state. If the network is in the DOWN state, raises a warning, and if another problem occurs (problem with parsing networks from OpenStack output, etc.), raises a critical error.

After verifying the correctness of my approach, I will provide more information about other checks.

WIP PR at https://github.com/juju-solutions/charm-openstack-integrator/pull/43

In my design, I changed only one thing, and that was creating individual py files for checks to reuse functions.

The approach described in #1 has been slightly modified. When a Neutron port reports "DOWN", the nagios alert raised is CRITICAL, not warning.

The PR mentioned in #2 is ready for review. As mentioned in my last comment [1], I think the use of python-openstackclient, installed via layer.yaml, will need to be reviewed. The nrpe script(s) are able to use native python libs, but it would break the approach taken until now (use a snap from the snapstore of deployed via Juju resources, in case no Internet access exists).

1. https://github.com/juju-solutions/charm-openstack-integrator/pull/43#issuecomment-778240754

The bug would not be complete, in case of approval of the previous PR, because a LB nrpe check is still missing (WIP).

After the discussion, we decided that NRPE checks should not be a
part of charm-openstack-integrator. Instead,
charm-openstack-service-checks should be used. Since it already has
check for Octavia LB, I will add only check for OpenStack resources.