ssh pool connections dont timeout

[Avishay Traeger]

In the SSHPool in utils.py, function create():
            transport.sock.settimeout(None)
            transport.set_keepalive(self.conn_timeout)

These two lines cause the SSH connections to remain open, which has two undesired side effects:
1) Unit tests that use SSH hang because SSH connections are not cleaned up
2) SSH connections remain open 24x7, even during idle times

[Avishay Traeger (avishay-il)]

Addressed by:
https://review.openstack.org/#/c/21358/