mgr/diskprediction_cloud: refactor timeout() decorator

mgr/diskprediction_cloud: refactor timeout() decorator

* timeout() is never passed any parameter when being called, so let's
  remove the parameters list of "seconds" and "error_message"
* use `getattr()` instead of `hasattr()` for retrieving the
  member variable of `self`
* pass `self` to wrapper function explicitly.
* return `func()` right away.
* hardwire the error message of `TimeoutError` to "Timer expired",
  because
  - as neither errno.ETIME nor errno.ETIMEOUT is portable
  - the only caller of `TimeoutError` is `timeout()`, so there is
    no need to have the flexibility to pass a different error message
* use `wraps()` as a decorator, simpler this way.

Signed-off-by: Kefu Chai <kchai@redhat.com>