Storage Config: Use own section

Storage Config: Use own section

Instead of configuring storage X by parameters in the main
section "X_class = backend" and "X_param = value", use a
new section in the config: "[storage:X]" and use "class =
backend" and "param = value" there.

This is the beginning, it includes a try at being backward
compatible. But that try isn't really fully useful anyway.