Moving eventlet.hubs.use_hub call up

Under the design concept of eventlet, we need to call
eventlet.hubs.use_hub() before the application starts
doing any I/O. And it makes custom log handlers register
correct event listeners.

Closes-bug: 1247889
Change-Id: I3d43021c8cb6ed630123781f3686d62e5159edfc
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Moving eventlet.hubs.use_hub call up"