sanity: allow Volume capacity_bytes to be zero

[NicolasT]

What type of PR is this?
/kind bug

What this PR does / why we need it:
From the CSI spec:

The capacity of the volume in bytes. This field is OPTIONAL. If not
set (value of 0), it indicates that the capacity of the volume is
unknown (e.g., NFS share).
The value of this field MUST NOT be negative.

As such, the tests that validate the result of a CreateVolume
operation shall accept zero as the capacity_bytes of the returned
Volume object, when validating this value against the requested
capacity.

See: https://github.com/container-storage-interface/spec/blob/e129a75169c13bade7c7336afe21f259740ba679/spec.md#createvolume

Which issue(s) this PR fixes:

Related to #94 but not fixing it completely (cfr. #263).

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

The tests that validate the result of a `CreateVolume` operation now accept zero as the `capacity_bytes` of the returned `Volume` object, when validating this value against the requested
capacity.

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[NicolasT]

/assign @jsafrane

[msau42]

/ok-to-test

[msau42]

/lgtm
/approve

Thanks for the fix! One last thing, can you add a release note for this fix? You can "/hold cancel" afterwards.
/hold

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: msau42, NicolasT

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[NicolasT]

Added a relnotes entry. Also, I removed the Fixes marker for #94 since this PR doesn't fix the whole issue (#263 plays a role as well).

/hold cancel