This is just a refactor, no behavior change. Instead of stitching together dhcpserver info in the structure returned from `read_host_only_interfaces`, sprout a new driver method called `read_dhcp_servers` to return that information separately. This means that driver clients (well there's really only _one_ client in `ProviderVirtualBox::Action::Network`) have to do a bit more work to get interface and DHCP server information. But this gives us (a) a cleaner and more consistent driver interface and (b) groundwork for a fix for #3083, which will require interacting with DHCP servers outside of the context of host-only interfaces.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.