MAJ: DISCOVERY_ARP_IP should be defined per APN

[nadicek]

DISCOVERY_ARP_IP is now defined as a static variable, however as ARP is a LAN protocol, the ARP originator address should be set/generated per APN and should come from the APNs subnet.

• source: magic.py

[nadicek]

Quick fix done, now ARPs originator IP are derived from APN address, more implementation is needed to have a nice clean approach. Present implementation can have problems, when a broadcast address is derived from APN address (this is a nonsence, right?)