providers/virtualbox: cleanup default vbox dhcp server

fixes #3083

Detect the presence of the default DHCP server that comes in a fresh
VirtualBox install and clean it up to prevent it from colliding with
Vagrant-managed network config.

In order to accomplish this, we:

 - add a `remove_dhcp_server` call to the virtualbox driver
 - fix dhcp options parsing to allow `:dhcp_{ip,lower,upper}`
   configuration options to make it through (so a user can override the
   removal behavior with some explicit configuration)
 - add the full `:network_name` to the details returned from
   `:read_dhcp_servers`, so we can have a durable value to pass to
   `:remove_dhcp_server`

Note that we do have to eat one more `VBoxManage list dhcpservers` for
each network interface to support this, but this seemed like a nominal
cost