YAPF, take 2

[alok]

What do these changes do?

See #1971. This is a cleaned up version meant to apply cleanly onto the
current master branch.

In summary:

1. Makes yapf actually use the existing .style.yapf file like it's supposed to
2. runs linting in parallel
3. More robust to weird shell misfeatures like word expansion
4. Adds linting to pandas and RLlib since @robertnishihara mentioned we should probably start doing that at some point. Since this PR doesn't actually change the files, we can do the lint in a follow-up PR (like Python linting fixes. #2076 did).

Related issue number

#1971

[richardliaw]

So no other linting changes will be applied in this PR?

[alok]

No, that's it. I just applied the actual PR changes, and the additional one you wanted.

[AmplabJenkins]

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