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
25ff636ee2