Add waitgroup for worker goroutines in run method

[rhrmo]

What type of PR is this?

/kind feature

What this PR does / why we need it:
This PR adds a wait group for all workers in the controller.go Run() method. This can be used during shutdown to wait till all the workers are done. We need this for a new featuregate in csi-sidecars for releasing leases right after sigterm/sigint - this would be an example of a merged PR in external-attacher. We need to wait for workers to finish before shutting down a sidecar completely.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

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[jsafrane]

/ok-to-test

[jsafrane]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jsafrane, rhrmo

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