[DBClientGroup] Add a new stub class.

DBClientGroup groups subclasses based on DBClient and ensure that
they use the same DBRMS and the same database. As a result, a join
operaton between DBClient subclasses can be performed with any
problem.
[DBCGroupRegular] Add a new stub group.

DBCGroupRegular will be a DBClient group that is in charge of
DBClient subclasses used almost always such as Event, Trigger,
and Host, Servers.
[DBCGroupRegular] Implement an initization mechanism.
[DBClientHost] Add a new stub class.

This class handles data for Host management sub system.
[test/Helpers] Consolidate commonly used constants.
[testDBClientHost] Create a test case with a few basic tests.
[DBClinetHost] Define a new table: host_info.
Merge pull request #311 from project-hatohol/dbclientgroup