fetch metrics correctly from control plane instances

[upodroid]

What type of PR is this?

/kind bug

What this PR does / why we need it:

When we run scale tests with kops, we get the following error:

W0213 14:15:54.305851   57465 metrics_grabber.go:105] Master node is not registered. Grabbing metrics from Scheduler, ControllerManager and ClusterAutoscaler is disabled.

This happens because the controlplane instances are no longer called master-*, now we'll look for the control plane labels/taints to detect the control plane instances

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[k8s-ci-robot]

@upodroid: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-perf-tests-gce-master-scale-performance-100	2da8d13	link	true	/test pull-perf-tests-gce-master-scale-performance-100
pull-perf-tests-ec2-master-scale-performance-100	2da8d13	link	true	/test pull-perf-tests-ec2-master-scale-performance-100

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.

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[Qqkyu]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Qqkyu, upodroid
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