virtualbox: handle a list of bridged nics

[bergman]

This change allows you to specify multiple network interfaces to bridge to in the order you want them tried, picking the first match.

VAGRANTFILE_API_VERSION = "2"

Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
  # ...
  config.vm.network "public_network",
    bridge: ["en4: Thunderbolt Ethernet",
             "en6: Broadcom NetXtreme Gigabit Ethernet Controller",
             "en0: Wi-Fi (AirPort)"]
  # ...
end

[berendt]

This new behavior should be documented in website/docs.

[bergman]

@berendt I am not familiar with this project enough to know how differences between virtualization providers are handled. This change only applies to virtualbox and the documentation seem to make no difference, at least not in the file I thought was relevant website/docs/source/v2/networking/public_network.html.md. Should this feature be added to all other providers as well, documented as virtualbox only, or not documented at all?

[berendt]

@sethvargo @mitchellh I do not understand the review/merge policy of Vagrant. For some PRs it is necessary to add documentation, for some PRs it is not necessary (like this one). After the merge of this PR this feature is now undocumented at https://docs.vagrantup.com/v2/networking/public_network.html. How should a user now be aware of this new feature? At the moment he has to read the sources to find this feature. I think there are some more features not documented at the moment and I do not like this.

@bergman I am also not familiar with this project, sorry for not replying to your comment. I hoped that @sethvargo will take care of the comment...