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/interface.js

@@ -11,7 +11,6 @@
 //     for the other API calls we leave it to the backend to retry. this allows
 //     them to tune the concurrency based on failures/rate limits accordingly
 exports = module.exports = {
-    getBackupRootPath,
     getProviderStatus,
     getAvailableSize,
 
@@ -45,13 +44,6 @@ function injectPrivateFields(newConfig, currentConfig) {
     // in-place injection of tokens and api keys which came in with constants.SECRET_PLACEHOLDER
 }
 
-function getBackupRootPath(apiConfig) {
-    assert.strictEqual(typeof apiConfig, 'object');
-
-    // Result: path at the backup storage
-    return '/';
-}
-
 async function getProviderStatus(apiConfig) {
     assert.strictEqual(typeof apiConfig, 'object');