backup site: add contents

it is a json that can be one of the three:

* null - include everything
* include - only include these ids
* exclude - everything except these ids

src/test/backupsites-test.js

@@ -29,7 +29,8 @@ describe('backups', function () {
         config: { backupDir: '/tmp/boxtest2' },
         format: 'rsync',
         retention: { keepWithinSecs: 2 * 24 * 60 * 60 },
-        schedule:  '00 00 23 * * *'
+        schedule:  '00 00 23 * * *',
+        contents: null
     };
 
     const appBackup = {