[CWL Executor] use dot repr flag of cwltool

[ryuwd]

Here was a comment, which I think was not addressed in the previous PR:

Out of curiosity, have you noticed that cwltool was providing a command to print a dot representation of the workflow?
I wonder whether printing and displaying this dot file using a python library would simplify the function.

Originally posted by @aldbr in #94 (comment)