Add volume size also to Volume status

[arttor]

Summary

Add size parameter to Volume status indicating the actual resources of the underlying volume. Size parameter in the spec will reflect requested resources.

Basic example

apiVersion: storage.onmetal.de/v1alpha1
kind: Volume
...
status:
  resources:
    storage: 100Gi

Motivation

Api user should know the actual size of the underlying volume. It may be different from requested size during volume resize operation. This can happen if the storage cluster doesn't have enough capacity to fulfill the new size.

[adracus]

We'll do this later when we handle storage resize correctly.