Use IAM role for AWS

Not 100% sure if this belongs here, perhaps this issue could be referenced on a different level (graylog2-docker, or graylog2-server).

Whenever possible, we shouldn't store any credentials like `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY`. For example, in the case of `provider=aws`, we should use IAM roles instead, by rendering a SA with the following annotation:

```yaml
apiVersion: v1
kind: ServiceAccount
metadata:
name: graylog-sa
annotations:
eks.amazonaws.com/role-arn: arn:aws:iam:::role/graylog-s3-role
```

This way, we are able to reduce the attack surface by not storing keys at all.