Sync common db code from Oslo

This sync contains commit - ce69e7f Don't store engine instances in oslo.db -
which removed global engine from oslo.db code. So, added code
for work with sqla engines and sessions to Ironic - get_engine()
and get_session() functions in ironic.db.sqlalchemy.api module.

Also this remove database slave connection and tpool features, because
they was removed from oslo code.

Latest oslo.db commit - 7959826af1f36a4bbc4c466d0d1e0b9efd468649

Change-Id: I14c3b823a8b975d5cd097778f3514cfb1abe2e10
Merge "Sync common db code from Oslo"