backups: add setup/teardown

1. add setup, teardown hooks
2. move the managed mount setup and teardown to filesystem backend
3. remove this vague storage.js

we should convert storageApi into a real object, so we don't have to
keep passing apiConfig around

src/apps.js

@@ -181,7 +181,6 @@ const appTaskManager = require('./apptaskmanager.js'),
     semver = require('semver'),
     services = require('./services.js'),
     shell = require('./shell.js')('apps'),
-    storage = require('./storage.js'),
     tasks = require('./tasks.js'),
     tgz = require('./backupformat/tgz.js'),
     TransformStream = require('stream').Transform,
@@ -2838,7 +2837,7 @@ async function getBackupDownloadStream(app, backupId) {
 
     const ps = new PassThrough();
 
-    const stream = await storage.api(backupTarget.provider).download(backupTarget.config, tgz.getBackupFilePath(backupTarget, backup.remotePath));
+    const stream = await backupTargets.storageApi(backupTarget).download(backupTarget.config, tgz.getBackupFilePath(backupTarget, backup.remotePath));
     stream.on('error', function(error) {
         debug(`getBackupDownloadStream: read stream error: ${error.message}`);
         ps.emit('error', new BoxError(BoxError.EXTERNAL_ERROR, error));