Cumulus_nvue: basic lag support

[jbemmel]

• Enables bonded interfaces with dynamic (LACP) or static modes
• Refactor IP address config as macro; used only in 'initial' script
• Explicitly disable ipv6 when not configured
• Define MTU in lag OSPF test such that it works on libvirt too

Note: Fixes ipv6-only unnumbered config, would have been skipped prior to this PR