Use explicit storageclass in all of our manifests

[tnozicka]

Using a "wrong" storageclass happens too often. Also in production, users would specify the storage class explicitly, instead of making our storageclass to be the default for everything. (Cost-wise it's not needed for other apps.)

Let's wire all of our manifests and examples to use scylladb-local-xfs storageclass which makes it less error prone for us and our users.

CI wise, for cases where want to test a setup with different storageclass and alias may be a viable option without the need to edit all the configs in every CI job.