Fix a bug that would cause a crash if StorageClassName was left to nil

[burmanm]

What this PR does:

• Changes handling of PersistentVolumeClaimSpec's *string in storageExpansion() method.

Which issue(s) this PR fixes:
Fixes #687

Checklist

• Changes manually tested
• Automated Tests added/updated
• Documentation added/updated
• CHANGELOG.md updated (not required for documentation PRs)
• CLA Signed: DataStax CLA

[garrynigel]

thanks @burmanm , Could we escalate and get this merged quickly?

[olim7t]

[question] Are we moving this inside the loop on the assumption that volume expansions will be rare?

[burmanm]

Yes and this check is irrelevant in our other use cases right now.

[garrynigel]

@burmanm thanks for merging this!! But I noticed in storage classes provided by cloud provider the default value is stored in annotations and not labels, so this always returns empty in my case.

See https://kubernetes.io/docs/tasks/administer-cluster/change-default-storage-class/#changing-the-default-storageclass

I modified the lines 166 to 175 slightly and only then it seems to be working.

	var defaultScName string
	var defaultScCount int
	
	if err := r.List(ctx, storageClassList); err != nil {
		return err
	}
	for i := range storageClassList.Items {
		storageClass := storageClassList.Items[i]
		if storageClass.Annotations != nil && storageClass.Annotations["storageclass.kubernetes.io/is-default-class"] == "true" {
			defaultScName = storageClass.Name
			defaultScCount++
		}
	
	if defaultScCount == 0 {
		return fmt.Errorf("no default storage class found, please specify StorageClassName in the CassandraDatacenter spec")
	} else if defaultScCount > 1 {
		return fmt.Errorf("found multiple default storage classes, please specify StorageClassName in the CassandraDatacenter spec")
	}