Release 12.0

[scottilee]

Release 12.0 introduces several changes and is compatible with Kubernetes v1.16.

[x] Updated the constants version
[x] Generated client changes
[x] Applied hotfixes
[x] Updated python-base submodules
[x] Updated the CHANGELOG

Fixes #1052

[k8s-ci-robot]

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[palnabarun]

A few comments from my side:

1. Did you update the constants.py with new values? I see that the KUBERNETES_BRANCH value is still release-1.15.
2. Maybe we should update the base submodule on master itself and it will get merged in with the master merge step. Otherwise, we will have an inconsistent master wrt the submodule.

Any thoughts? @roycaihw @yliaog

[palnabarun]

/cc

[scottilee]

New PR will be posted.