Add client_socket_timeout option

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

Change-Id: If492810a2f10fa5954f8c8bb708b14be0b77fb90
Closes-bug: #1371022
Merge "Add client_socket_timeout option"