There are two ways to give other greenthread chance to run:
greenthread.sleep(0) or time.sleep(0). Add the second way in
threading.rst and recommend the first way for contributors.
Also, PyMySQL works well with eventlet [1], and it's the default MySQL
DB API driver for oslo.db now[2].
So adjust 'threading model' doc with such info.
[1] https://wiki.openstack.org/wiki/PyMySQL_evaluation
[2] http://docs.openstack.org/developer/oslo.db/installation.html
Change-Id: Ifcf48cddcf52ce720ceb275cc3eb4d6be61a3ca5
