rgw: admin ops create user API can not determine existing user

Fixes: #8583

Signed-off-by: Ray Lv <raylv@yahoo-inc.com>
Merge pull request #1950 from xylv/wip-8583

rgw: admin ops create user API can not determine existing user

Reviewed-by: Yehuda Sadeh <yehuda@redhat.com>