AWS Provisioner does not work correctly with Machine_batch

If you create a series of machines surrounded with a machine_batch, they will not be created in parallel. The allocate_machine method will be executed from multiple threads and will fail with Argument errors on actual_instance.exists?

A quick solution was to add in a mutex to the method. It would still not be ran in parallel but would create safety to avoid failures.

After discussion with @johnewart, he decided that the best solution was to have the aws driver implement it's own version of allocate_machines that accepts and uses the cheffs parallelizer correctly