always remove host-only DHCP servers

A fresh install of VirtualBox automatically creates a host-only
interface (i.e., vboxnet0), with the DHCP server enabled.  This change
ensures that all DHCP servers are removed whether using Vagrant or
VirtualBox directly.

Fixes #39.

Conflicts:

	vbox_create.sh
validate VirtualBox environment & bump min version

Introduce virtualbox_env.sh to verify that `VBoxManage` is available,
that VirtualBox meets the minimum supported version, and set the
variable VBM as a shorthand for `VBoxManage`.  If `VBoxManage` is not
available, then exit.  Otherwise, if the detected version does not meet
the minimum supported version, a warning is emitted.

Also, the minimum supported version is now bumped to be >= 4.3.x.

Conflicts:

	non_vagrant_boot.sh
Fix Issue #94 - VirtualBox Version Check Fails
Removing error checking; fixing immediate issue
Do not inconvinence other VirtualBox DHCP servers
Fix for Changes needed for single-homed hosts