Moves Hashi Vault client login to use `auth.approle.login` as
current method is being deprecated in the next release.

```
DeprecationWarning: Call to deprecated function 'auth_approle'.
This method will be removed in version '0.12.0' Please use
the 'login' method on the 'hvac.api.auth_methods.approle'
class moving forward.
client.auth_approle(vault_role_id, vault_secret_id)
```

Change-Id: Ie5c1ebe99c8508336cc10944fdaa742ad7d1d85e