rgw: link account root to account user index

rgw: link account root to account user index

account root users were not linked to the account's user index because
they're not visible to iam apis like ListUsers

but now that 'account rm' is prevented from deleting the account while
users are still present, we want account root users to prevent deletion
too

add root users back to the account user index, but filter them out of
the iam user apis

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit b60335997266932f0a2c26fefc128732c66551a1)