db.sqlalchemy.session add [sql].idle_timeout

Keystone is using idle_timeout in the [sql] section for the
idle timeout. Before Keystone can use oslo-incubator's
db.sqlalchemy.session we'll need to be able to have the old
option still be usable but deprecated.

bp use-common-oslo-db-code

Change-Id: Ic3dd4ec7f678f9564597830f9e4764d979a5d18a
Merge "db.sqlalchemy.session add [sql].idle_timeout"