Backport 674: Adding validation for default snapshot classes per driver

[shawn-hurley]

What type of PR is this?

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/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
Adding validation so that a user can not create or update to a situation where they have more than default VolumeSnapshotClass

Which issue(s) this PR fixes:

Special notes for your reviewer:

backport of #674

Does this PR introduce a user-facing change?:

Cherry-pick #674: snapshot-validation-webhook: Adding validation for VolumeSnapshotClass to only have a single default for a particular driver. 

[k8s-ci-robot]

Hi @shawn-hurley. Thanks for your PR.

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[xing-yang]

/ok-to-test

[xing-yang]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: shawn-hurley, xing-yang

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