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
Expounded on global environments.

Explained that the effective environment used for a stack is the
combination of the user-supplied environment and the provider-supplied
"global" environment.

Change-Id: I0e005dad6efb80a76ecff6337454e2e891e692ca
Partial-Bug: #1297060
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)"
Merge "Expounded on global environments."