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>
(cherry picked from commit 180f0b7b66a37e4aa5221e68882d009bed686b58)