Assume that oauth2 DB configuration is in a subkey

- i.e.:

```php
[
    'zf-mvc-auth' => [
        'authentication' => [
            'oauth2' => [
                'db' => [
                    /* dsn, username, password */
                ],
            ],
        ],
    ],
]
```
Merge branch 'hotfix/oauth2-config'

Fixes to oauth2 configuration