feat: de-dupe KubeletTooManyPods, add cluster to descriptions

[skl]

This PR has two sets of changes:

• Adds the cluster label to any alert description where it was missing (currently inconsistent)
• More robust de-dupe of pod count in KubeletTooManyPods alert

Fixes #997.

[skl]

Here is the "More robust de-dupe of pod count in KubeletTooManyPods alert".

[skl]

btw this change is covered by an existing test:

kubernetes-mixin/tests.yaml

Lines 406 to 435 in 03c13f9

	- interval: 1m
	input_series:
	- series: 'kube_node_status_capacity{resource="pods",instance="172.17.0.5:8443",cluster="kubernetes",node="minikube",job="kube-state-metrics",namespace="kube-system"}'
	values: '3+0x15'
	- series: 'kube_pod_info{endpoint="https-main",instance="172.17.0.5:8443",job="kube-state-metrics",cluster="kubernetes",namespace="kube-system",node="minikube",pod="pod-1",service="kube-state-metrics"}'
	values: '1+0x15'
	- series: 'kube_pod_status_phase{endpoint="https-main",instance="172.17.0.5:8443",job="kube-state-metrics",cluster="kubernetes",namespace="kube-system",phase="Running",pod="pod-1",service="kube-state-metrics"}'
	values: '1+0x15'
	- series: 'kube_pod_info{endpoint="https-main",instance="172.17.0.5:8443",job="kube-state-metrics",cluster="kubernetes",namespace="kube-system",node="minikube",pod="pod-2",service="kube-state-metrics"}'
	values: '1+0x15'
	- series: 'kube_pod_status_phase{endpoint="https-main",instance="172.17.0.5:8443",job="kube-state-metrics",cluster="kubernetes",namespace="kube-system",phase="Running",pod="pod-2",service="kube-state-metrics"}'
	values: '1+0x15'
	- series: 'kube_pod_info{endpoint="https-main",instance="172.17.0.5:8443",job="kube-state-metrics",cluster="kubernetes",namespace="kube-system",node="minikube",pod="pod-3",service="kube-state-metrics"}'
	values: '1+0x15'
	- series: 'kube_pod_status_phase{endpoint="https-main",instance="172.17.0.5:8443",job="kube-state-metrics",cluster="kubernetes",namespace="kube-system",phase="Running",pod="pod-3",service="kube-state-metrics"}'
	values: '1+0x15'
	alert_rule_test:
	- eval_time: 10m
	alertname: KubeletTooManyPods
	- eval_time: 15m
	alertname: KubeletTooManyPods
	exp_alerts:
	- exp_labels:
	cluster: kubernetes
	node: minikube
	severity: info
	exp_annotations:
	summary: "Kubelet is running at capacity."
	description: "Kubelet 'minikube' is running at 100% of its Pod capacity."
	runbook_url: https://github.com/kubernetes-monitoring/kubernetes-mixin/tree/master/runbook.md#alert-name-kubelettoomanypods