backups: add remotePath

the main motivation is that id can be used in REST API routes. previously,
the id was a path and this had a "/" in it. This made /api/v1/backups/:backupId
not work.

src/storage/s3.js

@@ -1,7 +1,7 @@
 'use strict';
 
 exports = module.exports = {
-    getBasePath,
+    getRootPath,
     checkPreconditions,
 
     upload,
@@ -92,7 +92,7 @@ function getS3Config(apiConfig, callback) {
 }
 
 // storage api
-function getBasePath(apiConfig) {
+function getRootPath(apiConfig) {
     assert.strictEqual(typeof apiConfig, 'object');
 
     return apiConfig.prefix;