Avoid warning in checking config classes in setup

Summary:
Current code emits warning for classes with constructors with parameters.
It also creates the objects which is bad if constructors do some actual work.

Test Plan:
Run setup with:

• correct classes
• not-existing class
• class with private constructor
• class not implementing the required class

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2056