core: cleanup in MoveOrCopyDiskCommand

- simplify canDoAction checks by calling CommandBase#failCanDoAction
- replace if-else statement with ternary if
- rename fields that are used for caching to being with 'cache' prefix
- remove the canDoActionMessages parameter from checkImageConfiguration
  method, the messages list is fetched inside the method instead

Change-Id: Id28853bca622bf19ef66c983fa2d7ea38b943fa7
Signed-off-by: Arik Hadas <ahadas@redhat.com>