move database config items into quantum.conf

Bug #1194784

This patch syncs all the DB configuration items into
quantum.conf and removes the related items of plugins' config
file to avoid duplication.

sqlite related sqlite_db and sqlite_synchronous are defined
in 'default' section, which I think is a defect. And it seems
quantum does not use sqlite_db item.

Change-Id: Ia47102ced0e4169f45f10b85311f298d8c026d31