Set Magnum cluster create timeout to 30 minutes

The rally tasks for Magnum time out after a default of 40 minutes, so they try to delete the cluster while it is still in CREATE_IN_PROGRESS. It is possible that this is why some clusters are not deleted properly, as the default cluster timeout in Magnum is 60 minutes.

A cluster that is still being created after 30 minutes is probably going to go all the way and fail at 60 minutes anyway so this shouldn't introduce any new failures.