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.

Add a parameter to take advantage of the new(ish) eventlet socket timeout
behaviour. Allows closing idle client connections after a period of
time, eg:

$ time nc localhost 8776
real    1m0.063s

Setting 'client_socket_timeout = 0' means do not timeout.

DocImpact:
Added wsgi_keep_alive option (default=True).
Added client_socket_timeout option (default=900).

SecurityImpact

Closes-Bug: #1361360
Change-Id: I399b812f6d452226fd306c423de8dcea8520d2aa
Merge "Eventlet green threads not released back to pool"