fix behaviour of vm with ifup failures
Issue Background
'vagrant up' still fails for bwh when running 'ifup' in the VM. The 'ifup' and 'ifdown' commands used to return 0 in most failure cases (!), but starting with ifupdown 0.8 (which is in Debian "stretch") most failures are properly reported by returning >0. Vagrant 1.6 seems to depend on the old behaviour but I'm guessing Vagrant 1.7 ignores failure of 'ifup' (?!). I have a workaround for this (install a wrapper script for ifup/ifdown) but there may be a better solution. (I tried downgrading ifupdown, but that would have broken many other packages.)