Add post processing to files generated by Python generators

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Allow users to define the environment variable "PYTHON_POST_PROCESS_FILE" to post process "py" files generated by Python generators, e.g. export PYTHON_POST_PROCESS_FILE='/usr/local/bin/yapf -I'

Please refer to 66deaf14 for the code changes done by yapf as an example.

cc @taxpon (2017/07) @frol (2017/07) @mbohlool (2017/07) @cbornet (2017/09) @kenjones-cisco (2017/11)