Make the verifyDnsConfig() api return the valid credentials

src/dns/caas.js

@@ -118,5 +118,9 @@ function verifyDnsConfig(dnsConfig, domain, ip, callback) {
     assert.strictEqual(typeof ip, 'string');
     assert.strictEqual(typeof callback, 'function');
 
-    return callback(null);
+    var credentials = {
+        provider: dnsConfig.provider
+    };
+
+    return callback(null, credentials);
 }