Cumulus_nvue: Add loopback and VRF support, fix IPv6

[jbemmel]

• Removes interface type test
• Puts interfaces in the correct VRF
• Fixes IPv6 to send out router-advertisements ( "off" means enable them ... )
• Updates docs (note I could not find bandwidth setting in NVUE)

Note that Cumulus supports multiple loopbacks by putting multiple IPs on a single loopback device; set commands are cumulative and don't replace existing addresses

This also means applying Netlab configs to a system that already has some existing configs won't replace those; this would require an 'unset' command

[ipspace]

Thanks for focusing on the NVUE loopbacks. However, I think we might still have a problem if someone configures more than one loopback per VRF (for example, to generate more prefixes)