Remove 15-second sleep from LUInstanceCreate

Remove 15 second sleep when wait_for_sync is not set. LUInstanceCreate already
calls _WaitForSync with oneshot=True, which already performs an internal
wait-loop for disks to start syncing.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>