[DBClientHost] Change the file name related with DBClientHost.
[Params] Change the constant name: DB_DOMAIN_ID_HOST.

DB_DOMAIN_ID_HOST ->
DB_TABLES_ID_HOST.
[DBClientHost] Rename a class: DBClientHost.

DBClientHost ->
DBTablesHost

This patch only changes the class name. The logic is unchanged.
[DBTablesHost] Inherit DBTables.
[DBCGroupRegular] Remove the class that is no longer used.
[DBHatohol] Don't have DBTablesConfig and DBTablesUser instances temporarily

When a DBHatohol instance is created, their constructors also run
and fails to connect the DB because of the lack of DB setup.
In the near term, I'll fix the DB setup mechanism. Now I add
an tentative fix to avoid the problem.
[DBClientGroup] Remove the unused class.
Merge pull request #432 from project-hatohol/new-db-tables-host-2

New db tables host 2