move wellKnownJson to domains

after some more thought:
* If app moves to another location, user has to remember to move all this config
* It's not really associated with an app. It's to do with the domain info
* We can put some hints in the UI if app is missing.

part of #703

src/test/reverseproxy-test.js

@@ -18,7 +18,8 @@ const DOMAIN_0 = {
     provider: 'noop',
     config: {},
     fallbackCertificate: null,
-    tlsConfig: { provider: 'fallback' }
+    tlsConfig: { provider: 'fallback' },
+    wellKnown: null
 };
 
 let AUDIT_SOURCE = { ip: '1.2.3.4' };