fix swift path issues with list_directory

There was an issue with the swift backend where it was listing the contents of the directory with only the shortnames. For example, if you tried to run the following: 

```
$ PYTHON_PATH=lib SETTINGS_FLAVOR=swift python
> from storage.swift import SwiftStorage
> import config
> 
> cfg = config.load()
> store = SwiftStorage(cfg)
> for file in store.list_directory(store.tag_path(namespace, repository)):
>   print file
```

you would get a list like this:

```
_index_images
tag_latest
```

This was causing an issue whenever you tried to run a `docker pull` as it would try to grab the data from the route `tag_latest`, instead of `repositories/<namespace>/<repository>/tag_latest`. This PR fixes that, and the paths will now be properly listed as

```
repositories/<namespace>/<repositories>/_index_images
repositories/<namespace>/<repositories>/tag_latest
```