rgw-admin: create subuser if needed when creating user

Fixes: #10103
Backport: firefly, giant
This turned up after fixing #9973. Earlier we also didn't create the
subuser in this case, but we didn't really read the subuser info when it
was authenticating. Now we do that as required, so we end up failing the
authentication. This only applies to cases where a subuser was created
using 'user create', and not the 'subuser create' command.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>