Improve unit test coverage for OpenStack Nova database modules

openstack nova.db packages include database access and entity model classes. These classes are not tested well.
And, also not well cleared specification for database schema and entities.

test target package is following.

* nova.db
* nova.db.sqlalchemy
* nova.db.sqlalchemy.migrate_repo
* nova.db.sqlalchemy.migrate_repo.versions

We are intend to improvement C1 coverage and check some points.

1. Are there any test for parameter checking?
2. Are there any test for flags, object state checking?
3. Are there any test for database value checking?
4. Are there any test for valid exception handling?

And clear the specification, Database API's input and output by using testcases.