Blocks until snapshot is complete.

def WaitForSnapshotCompleted(snapshot):
  """ Blocks until snapshot is complete. """
  print 'Waiting for snapshot %s to be completed...' % (snapshot)
  while True:
      snapshot.update()
      sys.stdout.write('.')
      sys.stdout.flush()
      #print 'status is: %s' % snapshot.status
      if snapshot.status == 'completed':
          break
      time.sleep(5)
  return