Set the name of k8s cluster in Stackdriver sink.

[kawych]

Add a parameter to Heapster that allows to set resource label 'cluster_name' for metrics exported to Stackdriver.

[k8s-ci-robot]

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[k8s-reviewable]

This change is 

[piosz]

/lgtm
/approve

[kawych]

I've signed the cla.

[piosz]

closed/reopened to re-trigger cla verification

[k8s-ci-robot]

@kawych: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-heapster-e2e	cbc2b02	link	/test pull-heapster-e2e

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.