Rax username and api key

Adds support for the RAX_USERNAME & RAX_API_KEY env vars (same as the rax module). Useful when running on a build agent.
To remain consistent with the other auth calls (`pyrax.keyring_auth` and `pyrax.set_credential_file`), you should also pass `region=region` to `pyrax_set_credentials` such as:

```
pyrax.set_credentials(username, api_key, region=region)
```