cleanup and fix network support in slapos node format
CLEANUP:
- the bridge was never used anymore so remove it totally
- for KVM software release, we are using tap with their own routes (packets are routed directly through the tap device).
- reformat a bit the code to more readable
FIX:
- fix the IPv6 generation for taps : use the last IP of the subnetwork for tap itself and support different sizes of network