Get credentials before checking if the SnapshotClass exists

[huffmanca]

What type of PR is this?
/kind bug

What this PR does / why we need it:
If a CSIDriver requires credentials, but the VolumeSnapshotClass has been deleted, then the VolumeSnapshotContent cannot be deleted even though it contains a reference to the credentials in its own annotations.

This PR grabs the credentials instead of checking to see if the VolumeSnapshotClass exists, so that we can delete VolumeSnapshotContent objects if the secret annotation is on the VolumeSnapshotContent.

Which issue(s) this PR fixes:
Fixes #412

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[xing-yang]

This will affect snapshot creation. We need to have SnapshotClass for snapshot creation.

Instead of making changes to getCSISnapshotInput, we can make a change in deleteCSISnapshotOperation.
Replace the call _, snapshotterCredentials, err := ctrl.getCSISnapshotInput(content) with snapshotterCredentials, err := ctrl.GetCredentialsFromAnnotation(content) as we only need to get credentials from annotation at delete snapshot time.

[huffmanca]

Thanks for the quick review! I'll make the changes now.

[xing-yang]

Can you update the PR description?

[huffmanca]

This has been done.

[xing-yang]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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