Default to right status when viewing a config 'class' option

Summary: T2361

Test Plan:
Set value to metamta.mail-adapter and reload page.  Defaults to assigned value.
Performed same test with metamta.can-send-as-user to check that functionality is not broken for config 'boolean' options.

Reviewers: epriestley

Reviewed By: epriestley

CC: kwadwon, aran, Korvin

Maniphest Tasks: T2361

Differential Revision: https://secure.phabricator.com/D4881