Always use a poll timeout in the executor

This change allows to stop the eventlet executor without
using eventlet.kill.

And also, the kombu docs actually recommend that drain_events
by default have a 1 timeout.

Co-Authored-by: Joshua Harlow <harlowja@yahoo-inc.com>

Change-Id: I3a3919e38d08267439843a127346300fd2131540
Merge "Always use a poll timeout in the executor"