Fix incorrect check in aggregate/az test

This test currently does:

  try:
      self.api.update_aggregate(...)
  except exception.InvalidAggregateAction as e:
      msg = "host1 in avail_zone1,host2 in avail_zone1"
      self.assertTrue(e.format_message().find(msg))

There are two issues with this: 1) if update_aggregate() doesn't raise
an exception, the test will pass and 2) if exception doesn't contain
the message (which it doesn't, it's not the correct message) then
find() will return -1 and the assertion passes.

Fix both of these simply by using assertRaises()

Change-Id: I31edef2cfdd9ce748f19b09e93ae00a7437a52f5
Merge "Fix incorrect check in aggregate/az test"