Add possibility to suggest size from machine

[Gerrit91]

This helps to either specify more exact sizes or create initial sizes for a new machine type.

$ metalctl size suggest <machine-id>
constraints:
    - max: 8
      min: 8
      type: cores
    - max: 140000000000
      min: 100000000000
      type: memory
    - max: 150000000000000
      min: 100000000000000
      type: storage