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