Migrate MySQL configs to ::Bind

Summary: Doing this proactively now for a few sites in order for my bad ini node diff to work without a lot of test runner `.expect` changes.

Plus this is the right thing to do anyway.

Note: I removed the `IniSetting::Bind` line because that is now duplicative to the new `Config::Bind` line above it.

Reviewed By: @ptarjan

Differential Revision: D1547854
Remove ini refactoring parameter artificat

Summary: Before `FillInConstants` I had overloaded `Set` to the chagrin of some. I forgot to actually remove `constants_only` as a paramater to `Set`.

Reviewed By: @paulbiss

Differential Revision: D1548688