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/storage/s3.js

@@ -1,7 +1,6 @@
 'use strict';
 
 exports = module.exports = {
-    getBackupRootPath,
     getProviderStatus,
     getAvailableSize,
 
@@ -92,13 +91,6 @@ function getS3Config(apiConfig) {
     return credentials;
 }
 
-// storage api
-function getBackupRootPath(apiConfig) {
-    assert.strictEqual(typeof apiConfig, 'object');
-
-    return apiConfig.prefix;
-}
-
 async function getProviderStatus(apiConfig) {
     assert.strictEqual(typeof apiConfig, 'object');