Remove db, db.sqlalchemy from openstack-common.conf

Since Keystone switched to using oslo.db, the db and db.sqlalchemy
modules in oslo-incubator are no longer used, so remove them from
openstack-common.conf.

Then I did a re-sync with oslo-incubator commit caed79d8, which
removed common/context.py since it's not used anymore.

Change-Id: Ief0052e1ea0279118f0687c3a2a19c5ea53c6eb9
Merge "Remove db, db.sqlalchemy from openstack-common.conf"