Trivial rename to improve consistency within the unit tests. Typically
the base and utils modules imported by various tests are prefixed by
db_, obj_, etc. to differentiate them. This change adds a db_ prefix to
the ironic.tests.unit.db.base and ironic.tests.unit.db.utils imports
outside of ironic.tests.unit.db and submodules.
Change-Id: I39803c592e32e58ce9657ae70bcd45f7b5d51809