[SDK] Code style enforcement

[numerology]

As discussed offline, currently KFP does not adopt a consistent coding style on its Python side. This is not the best practice.

Here we propose to use yapf to help contributors to enforce Google python coding style. I will send out a PR for the yapf configuration. The default configuration does not work very well :(

Also, I propose to use pylint to check the coding style in Travis CI.

[Update 10/21] Since yapf does do much to docstring, docformatter is recommended to auto-lint the docstring if a contributor found it hard to pass pylint checks.

[numerology]

/area sdk
/priority p1

[numerology]

/area testing
/area samples

[eterna2]

I would love to have mypy included too.

[numerology]

I would love to have mypy included too.

Sounds good. However it seems like before we enforce mypy we need some additional works to make sure we can pass its tests

[Bobgy]

FYI, tracker issue for frontend part is #2406

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