Use LB server configuration to force DB domains in ExternalStorageDB
This is for backwards-compatibility for pre 14ee3f210782 external store
configuration that relied on not using the main wiki DB name(s).
(cherry-picked from 92e4ace7eae612ce0fa040e02d932516c4d99712)
Bug: T200471
Change-Id: Ie60cae64e32ff2532565cbd79c8e084634a61cce