Use oslo.db sessions

Keystone was using its own keystone.common.sql module for
database access. oslo-incubator's db.sqlalchemy module provides
the same or better functionality, so use that instead.

DocImpact
- The options that were in the [sql] section are deprecated and
  replaced by options in the [database] section. There are
  also several new options in this section. If database
  configuration is described for another project that uses
  oslo-incubator's db.sqlalchemy module the docs can be shared.

Part of bp use-common-oslo-db-code

Change-Id: I25b717d9616e9d31316441ae3671d2f86229c2bf
Merge "Use oslo.db sessions"