rgw: subuser creation fixes

Fixes: #8587
There were a couple of issues, one when trying to identify whether swift
user exists, we weren't using the correct swift id. The second problem
is that we relied on the gen_access flag in the swift case, where it
doesn't really need to apply.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge pull request #2564 from ceph/wip-8587

rgw: subuser creation fixes

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>