Adding Vultr Provider

[Oogy]

Adds discovery on Vultr Cloud.

[ddymko]

Hey there! @Oogy and I are from the Vultr open source team and wanted to have us integrate with go-discover.

Let us know if there anything that would be required or needed in addition to this pr.

[hashicorp-cla]

All committers have signed the CLA.

[endigma]

why is this blocked for 3 years?

[ddymko]

@endigma feels like this repo is no longer maintained

[endigma]

given it's pretty important part of consul bootstrapping maybe some activity would be nice

[endigma]

if go-discover isn't going to iterate at all, perhaps ya'll at vultr can make a CLI command that does exactly the same thing so one could use sed or whatever to get it done

filtering public/vpc addrs would also be useful

[heatzync]

I see there are some conflicts... Probably due to other changes that happened in the meantime while this pull request has been waiting to be merged.

[heatzync]

Seems https://github.com/hashicorp/go-netaddrs is now the way to go?