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