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/backuptask-test.js

@@ -29,8 +29,6 @@ describe('backuptask', function () {
             provider: 'filesystem',
             backupFolder: path.join(os.tmpdir(), 'backupstask-test-filesystem'),
             format: 'tgz',
-            retentionPolicy: { keepWithinSecs: 10000 },
-            schedulePattern: '00 00 23 * * *'
         };
 
         before(async function () {