config: upgrade password sync

Before this patch, user password could be set or updated only for
auth_type == 'chap-sha1'. Now password can be set, updated or removed
for any auth_type. Note that the password is changed only if necessary
to minimise db writes.

Part of #8967

NO_DOC=tarantool/doc#3544 links the most actual schema,
       no need to update the issue.