modify kube-prometheus-static-etcd.libsonnet servicemonitorEtcd namespace (#592)

* Change the servicemonitorEtcd namespace kube-system to $._config.namespace

* delete  Watchdog of alert rule

* Revert "delete  Watchdog of alert rule"

This reverts commit 815b922ead4db9e489419925b285705369757a71.

Co-authored-by: tyger <tyger.cheng@oriente.com>