Use strings in TestServer::Config
Use strings in TestServer::Config

Summary:
We need these to be writeable, and it's easier to use them if they are
strings.

Reviewed By: kevinlewi

Differential Revision: D7528287

fbshipit-source-id: 624c9b129474b9bd4fbefe565906d9002c078257

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter