mussel: add the new namespace "backup_storage".

#  `backup_storage` namespace

## Usage

```
$ mussel.sh backup_storage index
$ mussel.sh backup_storage show [uuid]
```

## Tests

### Unit tests

```
$ cd wakame-vdc/client/mussel

$ ./test/unit/v12.03/t.backup_storage.sh
$ ./test/component/v12.03/t.backup_storage.sh
```

### Integration test: connecting to dcmgr-api endpoint.

+ should be running dcmgr-api before running the integration test.

```
$ cd wakame-vdc/client/mussel

$ ./test/integration/v12.03/ro/t.backup_storage.sh
```