pybind: simplify timeout handling in run_in_thread()

pybind: simplify timeout handling in run_in_thread()

* no need to poll for thread exit, instead just use the timeout
  parameter of Thread.join()
* raise Exception("timed out") if times out
* do not catch KeyboardInterrupt in run_in_thread(), let
  the upper caller to take care of it. as run_in_thread()
  should always return the *retval* of the function, not a tuple
  representing an exception or failure.

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