[DB][DBAgentFactory] Move a function to create a DBAgent instance to a factory class.

Previously, a class: DB directly create a DBAgent instance. However,
I will add a feature to select the actual instance to be created by
a configuration file.

So the creation should be consolidated to a factory class.
Merge pull request #491 from project-hatohol/create-dbagent-by-factory-2

[DB][DBAgentFactory] Move a function to create a DBAgent instance to a f...