notifications: send backup fail only to owner

only superadmin has access to server and can adjust backup config

src/users.js

@@ -16,6 +16,7 @@ exports = module.exports = {
     getByResetToken,
     getByUsername,
     getAdmins,
+    getSuperadmins,
     setPassword,
     update,
     createOwner,
@@ -496,6 +497,16 @@ function getAdmins(callback) {
     });
 }
 
+function getSuperadmins(callback) {
+    assert.strictEqual(typeof callback, 'function');
+
+    userdb.getByRole(exports.ROLE_OWNER, function (error, owners) {
+        if (error) return callback(error);
+
+        callback(null, owners);
+    });
+}
+
 function sendPasswordResetByIdentifier(identifier, callback) {
     assert.strictEqual(typeof identifier, 'string');
     assert.strictEqual(typeof callback, 'function');