gen_app: Start with a removal of the OutputDir.. don't rely on cobra having run.
gen_app: Start with a removal of the OutputDir.. don't rely on cobra having
run.

This enables programmatic calls to the Generator.

ZjMNZHgG5jMXw/goa,goadesign/goa,dominicm/goa,raphael/goa,smessier/goa