Eventlet green threads not released back to pool

Presently, the wsgi server allows persist connections hence even after
the response is sent to the client, it doesn't close the client socket
connection.
Because of this problem, the green thread is not released back to the pool.

In order to close the client socket connection explicitly after the
response is sent and read successfully by the client, you simply have to
set keepalive to False when you create a wsgi server.

DocImpact:
Added wsgi_keep_alive option (default=True).
In order to maintain the backward compatibility, setting wsgi_keep_alive
as True by default. Recommended is set it to False.

Conflicts:
        cinder/wsgi.py
        etc/cinder/cinder.conf.sample

SecurityImpact

Closes-Bug: #1361360
Change-Id: Ic57b2aceb136e8626388cfe4df72b2f47cb0661c
(cherry picked from commit fc87da7eeb3451e139ee71b31095d0b9093332ce)