feature: configurable alerts

[tarokkk]

fixes: #1749
deprecate: #1775

This PR introduces a new type called PrometheusRulesOverride. The content of this type is identical to the v1.Rule Prometheus rule type. The controller will match overrides by their names with the original rules. All of the override attributes are optional and whenever an attribute is set it will replace the original attribute.

Example to change a rule severity:

fluentd:
  metrics:
    prometheusRules: true
    prometheusRulesOverride:
    - alert: FluentdPredictedBufferGrowth
      labels:
         rulegroup: fluentd
         service":   fluentd
         severity":  none