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
Remove unused variable for UpdateReplace exception

Base class Exception have not attribute "_message" and variable
"message" has default value in init method, so attribute "_message"
could be removed.

Change-Id: I304e13d1923ef043f04f03477b94f88cea4bc52f
Merge "Sort requirement files in alphabetical order"
Merge "Order imports in alphabetical order (5/9)"
Merge "Expounded on global environments."
Sync latest oslo.db code

Common db code was updated in oslo. The most important thing is that
engine instances don't stored anymore in oslo.db - ce69e7f.
This patch moves methods `get_engine` and `get_session` to module
`heat.db.sqalchemy.api`.

Latest commit in oslo related to db module:
2fd457bf2ccbeb2b84ffb204778b6417cd5405ba

Change-Id: Iaa2e9ba26e824c678c698914170e3dffbf1c5c95
Merge "Remove unused variable for UpdateReplace exception"