Add JcloudsLocation.MAX_CONCURRENT_MACHINE_CREATIONS

- Prevent more than the given number of concurrent calls to create
  VMs for a given JcloudsLocation instance.
- Required in various clouds (sometimes, at least!) such as aws-ec2
  where one gets http response 503 RequestLimitExceeded when trying to
  provision 18 machines concurrently.
Fix JcloudsLocationTest

- only throw NoMachinesAvailableException if the last obtain attempt
  threw that; otherwise throw the underlying exception.
- (the problem is probably not that the cloud is full with no more
  machines available!)