Reduce Prometheus addon's node-exporter tolerations

* Change node-exporter DaemonSet tolerations from tolerating
all possible NoSchedule taints to tolerating the master taint
and the not ready taint (we'd like metrics regardless)
* Users who add custom node taints must add their custom taints
to the addon node-exporter DaemonSet. As an addon, its expected
users copy and manipulate manifests out-of-band in their own
systems

addons/prometheus/exporters/node-exporter/daemonset.yaml

@@ -57,7 +57,9 @@ spec:
             mountPath: /host/root
             readOnly: true
       tolerations:
-        - effect: NoSchedule
+        - key: node-role.kubernetes.io/master
+          operator: Exists
+        - key: node.kubernetes.io/not-ready
           operator: Exists
       volumes:
         - name: proc