add option to ignore kube-system pods

[Stone Preston]

The kube system pod check is an issue for CAPI deployments as a CNI is not available, so many kube-system pods wont start until that is deployed. The check keeps the charm from going active idle. This pr: https://github.com/charmed-kubernetes/charm-kubernetes-control-plane/pull/280 adds a config option to ignore pods in kube-system, which can allow the charm to reach active/idle status even with some pending pods.