backups: make id, provider a backend specific setting

the backend can stash whatever values it wants in the config.
just like the DNS backends, we make verifyConfig return a sanitized config.
added benefit is that extra user fields (via API) are also not dumped into the db.

src/storage/noop.js

@@ -17,7 +17,7 @@ exports = module.exports = {
     teardown,
     cleanup,
 
-    testConfig,
+    verifyConfig,
     removePrivateFields,
     injectPrivateFields
 };
@@ -103,8 +103,12 @@ async function cleanup(apiConfig, progressCallback) {
     assert.strictEqual(typeof progressCallback, 'function');
 }
 
-async function testConfig(apiConfig) {
-    assert.strictEqual(typeof apiConfig, 'object');
+async function verifyConfig({ id, provider, config }) {
+    assert.strictEqual(typeof id, 'string');
+    assert.strictEqual(typeof provider, 'string');
+    assert.strictEqual(typeof config, 'object');
+
+    return {};
 }
 
 async function setup(apiConfig) {