rgw: evaluate_iam_policies() handles account root user

rgw: evaluate_iam_policies() handles account root user

> By default, all requests are implicitly denied with the exception of
> the AWS account root user, which has full access.

the account root user turns an implicit deny from identity policy into
an allow, though other policies can still deny explicitly

Signed-off-by: Casey Bodley <cbodley@redhat.com>