Hide Config::Load method

Constructor is going to load data instead.
This makes clear that ::Load is one time operations.

Change-Id: Id7b2275544cc034d30737bea21b3d8cc68e4a8a9
Reviewed-on: https://weave-review.googlesource.com/2080
Reviewed-by: Vitaly Buka <vitalybuka@google.com>