[cinder-csi-plugin] Use context

[stephenfin]

What this PR does / why we need it:

Use the generated VolumeContext.

Which issue this PR fixes(if applicable):
fixes #2863

Special notes for reviewers:

Release note:

NONE

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[stephenfin]

/cc @kayrus

...again since #2743 was your PR so I'd like to make sure I'm not missing anything obvious 😅