Sort requirement files in alphabetical order

This makes code more readable, and can check whether specific library
in the requirement files easily. We enforce the check in pep8, We also
add the tools for checking requirements.txt and test_requirements.txt.

Change-Id: Ia0a24553b7a5204faf6413c17dae6ba6aae2b5e8
Closes-Bug: #1285478
Merge "Sort requirement files in alphabetical order"