backend,frontend: reduce ExportRepoImage parameters

In this patch:
* reduce the number of required parameters for ExportRepoImageCommand
  to imageGroupId and destinationDomainId
* switch from StoragePoolValidator to StorageDomainValidator in the
  canDoAction (to handle also the domain in maintenance case)
* update the ExportRepoImage tests
* update the serialVersionUID for ExportRepoImageParameters as it was
  already in use by ImportRepoImageParameters

Change-Id: I166b19b9d69077da8e9ba4cb0d63fd31fdd4e45d
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>