Fix AssertionError and backtrace when kill congress-server
Fix AssertionError and backtrace when kill congress-server

The key problem is we use the same GreenPool for wsgi request threads.
GreenPool thread cannot wait for itself to terminate. This patch tells
evenlet wsgi server to create a new GreenPool for requests.

Change-Id: I8220223855a6a0347359260cdb91d970e0d12aae
Closes-Bug: #1416524

ekcs/congress,ekcs/congress,ekcs/congress,ramineni/my_congress,ramineni/my_congress,ekcs/congress,ramineni/my_congress,openstack/congress,ramineni/my_congress,openstack/congress