label cinder-csi storage class as 'is-default-class' only when relevant

I realized during the review of !1767 (merged) that we are currently always labeling the cinder-csi StorageClass with storageclass.kubernetes.io/is-default-class, even in the case where the code at https://gitlab.com/sylva-projects/sylva-core/blob/ffff511044f5bbb3ef13de555c7ac5462589e19a/charts/sylva-units/values.yaml#L5272 would elect a different CSI as the default storage class.

In practice the cases where this would happen are only hypothetical, but nonetheless setting this storageclass.kubernetes.io/is-default-class: "true" should really be only done conditionally.

/cc @mohan.l.sharma