Make ExternalStoreDB "wiki" context override the server "dbname" field
This is all that is needed for b/c and going beyond that can break
foreign wiki (domain) external store access if matching per-wiki DB
names *are* used for external store.
Follow up to 92e4ace7eae61.
Bug: T200471
Change-Id: I877139ff659f542da04d4d8f5ef3297dbfcfd734