Specify heap space parameters in Cromwell command

[vdauwera]

By default the memory allocation to Cromwell is small, and while this is fine for lightweight workflows, it's not sufficient for big workflows with wide scatters. Combined with using --memory 2 to request a bigger machine, this prevents Cromwell from dying.

Making the value larger than it strictly needs to be so we can scale up as needed with just the cmdline argument; it doesn't seem to hurt when running small workflows on a smaller machine.

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