fix error when --easystack is used without having PyYAML installed #3515
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This fixes the following ugly crash when trying to use
--easystackwithout having PyYAML installed (which is a problem I introduced with the commits I added to #3479 before merging it), and also improves the@only_if_module_is_availabledecorator a bit (which was the underlying cause of the trouble).closes #3514