rgw: RGWUser::init no longer overwrites user_id

rgw: RGWUser::init no longer overwrites user_id

if an admin op specifies a user_id and does not find a user with that
id, but does find a user based on a later field (email, access key,
etc), RGWUser::user_id will be overwritten with the existing user's id

when this happens on 'radosgw-admin user create', RGWUser::execute_add()
will modify that existing user, instead of trying to create a new user
with the given user_id (and failing due to the conflicting email,
access key, etc)

by preserving the original user_id (when specified), this uid conflict
is detected in RGWUser::check_op() and a "user id mismatch" error is
returned

Fixes: http://tracker.ceph.com/issues/21685
Signed-off-by: Casey Bodley <cbodley@redhat.com>