Prevent calling waitall() inside a GreenPool's greenthread

Prevent calling waitall() inside a GreenPool's greenthread

When neutron-server is running with several api workers
sending it a termination signal (SIGTERM, SIGHUP or SIGINT)
leads to waitall() being called inside a GreenPool's greenthread.

The reason is that a wsgi server is started in a green thread
from the same green pool that is passed to the server itself
to be used for spawning client green threads.

To avoid it, it is reasonable to use different pools for spawning
a wsgi server and for its internal usage.

This is also the case for metadata agent running with several
metadata workers.

Change-Id: I38174396f06fcb29ac0776534ac6494dabb00df6
Closes-Bug: #1423250