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/interface.js

@@ -11,7 +11,7 @@
 //     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 = {
-    getBasePath,
+    getRootPath,
     checkPreconditions,
 
     upload,
@@ -49,7 +49,7 @@ function injectPrivateFields(newConfig, currentConfig) {
     // in-place injection of tokens and api keys which came in with constants.SECRET_PLACEHOLDER
 }
 
-function getBasePath(apiConfig) {
+function getRootPath(apiConfig) {
     assert.strictEqual(typeof apiConfig, 'object');
 
     // Result: path at the backup storage