core: fix stateless flow in RestoreAllSnapshots

RestoreAllSnapshotsCommand -> restoreSnapshotAndRemoveObsoleteSnapshots:
* Global member 'imagesToRestore' should be assigned in STATELESS
  case as well.
* Decoupled PREVIEW and STATELESS cases for readability and
  easier maintenance.

Change-Id: I9aeb6ac37b511e0cd39b290f9571d0a73c7a5717
Bug-Uril: https://bugzilla.redhat.com/1057221
Signed-off-by: Daniel Erez <derez@redhat.com>