Improve csi-snapshotter ClusterRole

[ialidzhikov]

Is this a bug fix or adding new feature?
/kind cleanup

What is this PR about? / Why do we need it?
With v2.0.0 of kubernetes-csi/external-snapshotter, it is split into two component - the csi-snapshotter sidecar and the snapshot controller (ref kubernetes-csi/external-snapshotter#182). As the VolumeSnapshot watch is moved to the new snapshot controller, the csi-snapshotter sidecar ClusterRole no longer needs rules for volumesnapshots.
This PR adapts the csi-snapshotter sidecar RBAC according to https://github.com/kubernetes-csi/external-snapshotter/blob/v2.1.1/deploy/kubernetes/csi-snapshotter/rbac-csi-snapshotter.yaml.

What testing is done?

[k8s-ci-robot]

Hi @ialidzhikov. Thanks for your PR.

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[coveralls]

Pull Request Test Coverage Report for Build 1087

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.871%

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Totals
Change from base Build 1085:	0.0%
Covered Lines:	1411
Relevant Lines:	1789

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💛 - Coveralls

[leakingtapan]

/ok-to-test

[leakingtapan]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ialidzhikov, leakingtapan

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