Check ephemeral and swap size in the API

Validate that ephemeral and swap disks passed in trough the API are
within size limits for the given instance type. The validation is done
in the API layer.

Closes-bug #1218977

Change-Id: I96c6e651e4b221313c39dfc41e79d536585fb955
Merge "Check ephemeral and swap size in the API"