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>
(cherry picked from commit 1441ffe8103f03c6b2f625f37adbb2e1cfec66bb)