Make sure create_repo respect organization privacy settings

[Wauplin]

see slack thread (private)

Enterprise Hub allows organizations to force all repositories to be private. Since create_repo assumes that a repo is public when private=True is not explicitly passed, an error is thrown by the server

403 Forbidden: org-name forbids the creation of public models.

This is because private: False is passed by default (since #1001 😕). This PR updates this by making None the default value for private and send the value to the server only when explicitly set by the user. This should make the life of Enterprise Hub users much easier (today they need to explicitly pass private=True in their calls).

Note: tests are currently failing (need an update in staging org settings).
Note 2: we'll have to update many (most?) .push_to_hub signature in the HF ecosystem to also pass None whenever appropriate. I'll open PRs once that's merged.

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TIL: better listen to the boss #1001 (comment)

[HuggingFaceDocBuilderDev]

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[hanouticelina]

left some nit suggestions on the private argument docstring – otherwise lgtm! thanks!

[Wauplin]

Thanks for the suggestion :)
CI should be fixed by now (I've updated some mocked tests + server-side update is live). Will merge once it's green