Order imports in alphabetical order (5/9)

This patch is one in a series to re-enable H306 style check rule
(imports are in alphabetical order).
It touches engine files (without resources).

Implements: blueprint reduce-flake8-ignored-rules (partial)
Change-Id: Ifd36fc49199c48af891bd8e7cc2d2262a53032eb
Sort requirement files in alphabetical order

This makes code more readable, and can check whether specific library
in the requirement files easily. We also enforce the check in pep8.

Change-Id: I032c3c3d29522ce85f64b21d6428d975e46cc701
Closes-Bug: #1285478
Merge "Sort requirement files in alphabetical order"
Merge "Order imports in alphabetical order (5/9)"