Automatic merge from submit-queue (batch tested with PRs 42162, 41973, 42015, 42115, 41923)

add nrpe-external-master relation to kubernetes-master and kubernetes-worker

**What this PR does / why we need it**:

This PR adds an an nrpe-external-master relation to the kubernetes-worker, kubernetes-master and kubeapi-load-balancer charms. This is needed to monitor the state of the workers, the masters and the load-balancers via Nagios.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*:

https://github.com/juju-solutions/bundle-canonical-kubernetes/issues/165

**Special notes for your reviewer**:

Original work by @axinojolais in PR #40897. All I've done is squash commits on his behalf.

**Release note**:

```release-note
The kubernetes-master, kubernetes-worker and kubeapi-load-balancer charms have gained an nrpe-external-master relation, allowing the integration of their monitoring in an external Nagios server.
```