fix notice on construction without 'containerPaths'

fix notice on construction without 'containerPaths'

The comment block seems to indicate that containerPaths should only be used
for backwards-compatibility, which implies it is optional.

The following code generated a notice:

$fsBackend = new FSFileBackend( array(
	'name'        => 'foo',
	'lockManager' => 'fsLockManager',
	'basePath'    => '/tmp/foo',
);