rework backup root

notes:
* backup root cannot come from backend. for dynamic mounts backend cannot know where it is mounted
* backupConfig is 3 parts - format / mount / password . there is also this rootPath (which should not be in db)
* password should be stored separately in settings at some point
* format has to be passed along everywhere because we allow restore from  same backupConfig but different format. we do this by saving the format in the backups table

fixes #819

src/backupformat/tgz.js

@@ -24,8 +24,7 @@ function getBackupFilePath(backupConfig, remotePath) {
     assert.strictEqual(typeof backupConfig, 'object');
     assert.strictEqual(typeof remotePath, 'string');
 
-    const rootPath = storage.api(backupConfig.provider).getBackupRootPath(backupConfig);
-
+    const rootPath = backupConfig.rootPath;
     const fileType = backupConfig.encryption ? '.tar.gz.enc' : '.tar.gz';
     return path.join(rootPath, remotePath + fileType);
 }