Change ssh_utils parameter to correctly sends keepalive packets

The current implementation of ssh_utils never sends keepalive packets.
In ssh_utils.SSHPool, the socket timeout parameter is set to None
intending to keep ssh connections open.
However, when the parameter is set to None, ssh_utils does not run the
code to compare idle duration and keepalive interval.

This patch reverts the socket timeout parameter to default (0.1 sec).
The ssh_utils compares them every 0.1 seconds, and sends a keepalive
packet if idle duration > keepalive interval (= self.conn_timeout).

Change-Id: I8234083107207b9ebc0849947e8de92b5cf3e36e
Closes-Bug: #1673662

cinder/ssh_utils.py

@@ -136,16 +136,8 @@ class SSHPool(pools.Pool):
                 msg = _("Specify a password or private_key")
                 raise exception.CinderException(msg)
 
-            # Paramiko by default sets the socket timeout to 0.1 seconds,
-            # ignoring what we set through the sshclient. This doesn't help for
-            # keeping long lived connections. Hence we have to bypass it, by
-            # overriding it after the transport is initialized. We are setting
-            # the sockettimeout to None and setting a keepalive packet so that,
-            # the server will keep the connection open. All that does is send
-            # a keepalive packet every ssh_conn_timeout seconds.
             if self.conn_timeout:
                 transport = ssh.get_transport()
-                transport.sock.settimeout(None)
                 transport.set_keepalive(self.conn_timeout)
             return ssh
         except Exception as e: