add config.adminFqdn()

src/config.js

@@ -28,6 +28,7 @@ exports = module.exports = {
     // these values are derived
     adminOrigin: adminOrigin,
     internalAdminOrigin: internalAdminOrigin,
+    adminFqdn: adminFqdn,
     appFqdn: appFqdn,
     zoneName: zoneName,
 
@@ -155,6 +156,10 @@ function appFqdn(location) {
     return isCustomDomain() ? location + '.' + fqdn() : location + '-' + fqdn();
 }
 
+function adminFqdn() {
+    return appFqdn(constants.ADMIN_LOCATION);
+}
+
 function adminOrigin() {
     return 'https://' + appFqdn(constants.ADMIN_LOCATION);
 }