cmd/swarm: only use the default bootnodes on the default network id

[skylenet]

It looks like that we try to connect to these bootnodes even when using a different network id than the default (4).

We shouldn't do this because the bootnode will reject these peers:

DEBUG[10-04|13:17:44.377|p2p/server.go:766]                                              Removing p2p peer                        id=1f204e1e9878e7ef conn=inbound    addr=212.51.xx.zz:53568   peers=52 duration=1.847ms   req=false err="Handshake error: Message handler error: (msg code 0): network id mismatch 61 (!= 4)"

It also looks like that the bootnode will blacklist the incoming IP for a while (30seconds):

DEBUG[10-04|13:17:55.970|p2p/server.go:874]                                              Rejected inbound connnection             addr=212.51.xx.zz:53606   err="too many attempts"

The client connecting to the bootnode will simply get an EOF without knowing why he's not being able to connect:

TRACE[10-04|15:05:19.591|p2p/dial.go:290]                                                Dial error                               task="staticdial 1401ec8b40b23cb2 3.122.203.99:30301" err=EOF