PrometheusHighQueryLoad alert: use configured selector
PrometheusHighQueryLoad alert: use configured selector

Currently we're hardcoding `job="prometheus-k8s"` as selector. This
doesn't work if your prometheus is elsewhere. Fortunately we have
`prometheusSelector` in `$._config`  which all the other alerts use.

Use that here too.

Signed-off-by: Iain Lane <1b8e6b1ba75465f62e9cdfa8d566721103c12053@orangesquash.org.uk>

prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus