backups: split config and policy

keeping them together makes the test/validation quite complicated.
for example, when policy is changed, we test the storage backends

part of #817

src/test/storage-test.js

@@ -38,8 +38,6 @@ describe('Storage', function () {
             key: 'key',
             backupFolder: null,
             format: 'tgz',
-            retentionPolicy: { keepWithinSecs: 10000 },
-            schedulePattern: '00 00 23 * * *'
         };
 
         before(function (done) {