Declare merge keys for PrometheusRule alert patches

[solsson]

Allows Kustomize patchesStrategicMerge for example

apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
  name: kubernetes-mixin-alerts
spec:
  groups:
  - name: kubernetes-resources
    rules:
    - alert: CPUThrottlingHigh
      labels:
        severity: warning

See https://github.com/kubernetes-sigs/kustomize/blob/master/examples/customOpenAPIschema.md