need to introduce "default_storage_class" to allow end users to select default storage class

Summary

Since we do have many storage classes. Users should be given choice to select the default storage class.

We need to introduce a new sylva-units values key that would allow users to select default storage class

We need to evolve the code so that:

• if the user specified the new default_storage_class setting, then use it
• else...
  • build a list of all CSI units that are enabled (excluding local-path)
  • if this list contains a single item, then set default_storage_class to the storage class of this unit
  • if not, leave it undefined (or fail ?)

related references

Follow up issue for the discussion !3825 (comment 2483763211)

Details