providers/virtualbox: fix flaky test when rsync is not installed

test-only change

when rsync is not installed on the machine running the unit tests, the
prepare_nfs_settings tests end up calling the :nfs_installed capability
on the host, which fails on the fake host wired up in tests.

this adds some explicit stubbing to prevent the implicit assumption that
rsync is installed.
providers/virtualbox: extract reading dhcpservers from hostonlyifs

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.
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