Refactor Jenkins backend HTTP apis
Refactor Jenkins backend HTTP apis

Summary:
Rename '_get_raw_response' to '_get_text_response', since it returns response text.
Remove *args and *kwargs from '_get_*_response' methods, keeping only the params we use rather
than mostly wrapping `requests`.
Add docstrings to api methods.

Test Plan: Ran unit; passed with 100% diff coverage.

Reviewers: wwu

Reviewed By: wwu

Subscribers: changesbot, wwu

Differential Revision: https://tails.corp.dropbox.com/D101178

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