EventLogging role: set a local $wgEventLoggingSchemaApiUri

Having the default $wgEventLoggingSchemaApiUri point at metawiki may be
convenient, but it also means that browser tests for extensions that rely on
the EventLogging role are improperly isolated, having a dependency on an
external service. So point it at localhost instead.

Change-Id: Ife2f0933fd1ae17d1f527ba0d85957d9589f68b9