Tune tcp_keepalive_time for all nodes

In case of RabbitMQ crashes and closing network connection for all clients we would like to have shorten the keepalive sysctl settings otherwise it will still take ~2 hrs to terminate the connections.
```
echo "5" > /proc/sys/net/ipv4/tcp_keepalive_time
echo "5" > /proc/sys/net/ipv4/tcp_keepalive_probes
echo "1" > /proc/sys/net/ipv4/tcp_keepalive_intvl
```

`tcp_keepalive_time`

- the interval between the last data packet sent (simple ACKs are not considered data) and the first keepalive probe; after the connection is marked to need keepalive, this counter is not used any further 

`tcp_keepalive_intvl`

- the interval between subsequential keepalive probes, regardless of what the connection has exchanged in the meantime 

`tcp_keepalive_probes`

 - the number of unacknowledged probes to send before considering the connection dead and notifying the application layer 

http://tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/