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
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"