Summary:
The `configparser` is now too heavyweight. Some other crates (ex. io, auth,
revisionstore) just want to extract config values without complicated parsing /
remote hotfix logic.
Add a configmodel crate to satisfy the need.
Reviewed By: DurhamG
Differential Revision: D27641666
fbshipit-source-id: 26bd0b606ae3d286b3ec218927aef726d6802c63
