/v2/admin/members API should use JSON containers in response

We use JSON containers in the /v2/keys API, as well as other CoreOS-specific APIs (fleet). We should follow suit with the `GET /v2/admin/members` and `GET /v2/admin/members/<ID>` calls. For example, rather than this:

```
[ {...}, {...}, ...]
```

The response should look like this:

```
{"members": [{...}, {...}, ...]}
```