netdetect: detect all servers before network bench

This commit add a new program to automize the detection of available
server to perform a network bench.

It features two parts :
- client side
It generates keepalive message on a multicast adress containing all it's
IP adresses taken from hw.py

- server side
Collects all the keepalive message and save server's name (mac address)
and it's associated IP adresses

If with miss 2 consecutive keepalive, the entry is deleted (that's the
scrubbing process)
If no changes occurs within a defined time, the discovery process is
over.

The first server that complete the detection send its server list to the
other to insure all servers have the same list.

A typical output is :
[root@localhost src]# ./netdetect.py hw.py
Sending keepalive for 00:21:cc:d9:bf:26
Adding 00:21:cc:d9:bf:26
Sending keepalive for 00:21:cc:d9:bf:26
Received Keepalive from 00:21:cc:d9:bf:26
Adding d8:9d:67:1b:07:e4
Sending keepalive for 00:21:cc:d9:bf:26
Received Keepalive from 00:21:cc:d9:bf:26
Received Keepalive from d8:9d:67:1b:07:e4
Adding d8:9d:67:1a:d0:fc
Sending keepalive for 00:21:cc:d9:bf:26
Received Keepalive from 00:21:cc:d9:bf:26
Received Keepalive from d8:9d:67:1b:07:e4
Received Keepalive from d8:9d:67:1a:d0:fc
Sending keepalive for 00:21:cc:d9:bf:26
Received Keepalive from 00:21:cc:d9:bf:26
Received Keepalive from d8:9d:67:1b:07:e4
Received Keepalive from d8:9d:67:1a:d0:fc
Sending keepalive for 00:21:cc:d9:bf:26
Received Keepalive from 00:21:cc:d9:bf:26
Received Keepalive from d8:9d:67:1b:07:e4
Received Keepalive from d8:9d:67:1a:d0:fc
Sending keepalive for 00:21:cc:d9:bf:26
Received Keepalive from 00:21:cc:d9:bf:26
Received Keepalive from d8:9d:67:1b:07:e4
Received Keepalive from d8:9d:67:1a:d0:fc
Sending keepalive for 00:21:cc:d9:bf:26
Received Keepalive from 00:21:cc:d9:bf:26
Received Keepalive from d8:9d:67:1b:07:e4
Received Keepalive from d8:9d:67:1a:d0:fc
About to send the synthesis
Exiting Client, end of discovery
Received Synthesis
End of Discovery with 3 systems!
Exiting server
Synthesis result
Server d8:9d:67:1b:07:e4 -> 10.66.6.136
Server d8:9d:67:1a:d0:fc -> 10.66.6.249
Server 00:21:cc:d9:bf:26 -> 192.168.1.185 10.152.18.104