[Python][Client] pure library client package

Created by: rienafairefr

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\. -> Caveat I wasn't sure if I needed to commit the changes that happened after running /bin/python-petstore.sh, I commited them just in case
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language. @taxpon @frol @mbohlool @cbornet @kenjones-cisco

Description of the PR

A few times already, when using Swagger-codegen before also, I was thinking that all the supporting files for Python packages are a lot of the time unecessary. If you're building a package around auto-generated code, you'll be using it as a library and have your own setup.py, etc.

This PR adds a "onlyPackage" cli options ([...]generate [...] -l python -DonlyPackage [...]) (couldn't think of a better name, possibilities "libraryClient", "pureLibrary", ... IDK)

When this option is activated,

• docs and tests are generated in subfolders of the package itself
• README.mustache is generated to <packageName>_README.md to not overwrite the overarching project's README.md
• all the supporting file at the root of the project (git_push, requirements, setup.py) are not generated