clear backup progress when initiating backup

this ensures that tools can do:
1. backup
2. wait_for_backup

without the synchronous clear, we might get the progress state of
an earlier backup.

src/cloudron.js

@@ -549,6 +549,9 @@ function backup(callback) {
     var error = locker.lock(locker.OP_FULL_BACKUP);
     if (error) return callback(new CloudronError(CloudronError.BAD_STATE, error.message));
 
+    // clearing backup ensures tools can 'wait' on progress
+    progress.clear(progress.BACKUP);
+
     // start the backup operation in the background
     backupBoxAndApps(function (error) {
         if (error) console.error('backup failed.', error);