Duplicates in generated tables

In the table nova-db.xml, the backend and db_backend are duplicates in generated
tables. The db_backend should be deprecated.

See also the diff here:
https://review.openstack.org/#/c/34671/17/nova/openstack/common/db/api.py

The two backends should be distinguished.

Change-Id: I7b53a95a4d79bbe52041f39a06653e0a0fcb6523
Closes-Bug:#1226464
Merge "Duplicates in generated tables"