core: add disk profile to disk's commands

* Allow null disk profile id, if there's a single disk profile
in relevant storage domain (trivial case to avoid breaking current flow).
* Replace null disk profile with trivial one.
* Validate disk profile in all disk flows.
* Permission for disk profiles will be handled in a later patch,
currently we rely on storage domain permissions (no change in flow).

Change-Id: Ie7b6d977243cffc0bde772665ebaca47340075c6
Bug-Url: https://bugzilla.redhat.com/1085049
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>