providers/virtualbox: allow and prefer static guest IPs for NFS

Since vbox guest properties are proving to be less reliable than we had
hoped, bring back the static config parsing mechanism for finding a
guest IP to hand to NFS. If we find a static IP (or set of IPs) we'll
use that instead of trying to probe guest properties.

This retains NFS support for DHCP interfaces while regaining the
reliability that we previously had when static IPs were required.
guests/ubuntu: remove mount_nfs cap
core: ctrl-c no longer raises trap-context exception
synced_folders/nfs: lock around sudo input [GH-2680]
Add funtoo guest support.  Based heavily on https://github.com/mitchellh/vagrant/pull/1840
core: re-arrange some code to make things easier to read later
core: correct some doc comments
core: box add fails early if box already exists [GH-2621]
core: warn if no provider is specified that we can't check for it
core: undo last for now
providers/virtualbox: export after checking if package exists [GH-2380]
provisioners/chef-solo: delete data bag secret [GH-2712]
Merge pull request #2674 from phinze/static-nfs-guest-ips

core: problems with using VirtualBox guest properties for reading NFS Guest IP
Update CHANGELOG
v1.4.2
website/www: 1.4.2
up version for dev
Revert "Add funtoo guest support.  Based heavily on https://github.com/mitchellh/vagrant/pull/1840"

This reverts commit 3559c0808607e90237c3f86c750404b97e7a531c.
Add funtoo guest support. Based heavily on https://github.com/mitchellh/vagrant/pull/1840
:fireworks: New year! :fireworks: