feat(Vagrantfile): alter configuration to allow use as Docker host vm

turning off guest additions and vboxsf makes this possible, which is taken directly from the default docker host configuration: https://github.com/mitchellh/vagrant/blob/master/plugins/providers/docker/hostmachine/Vagrantfile
feat(channel): Allow user to configure CoreOS update channel
Merge pull request #102 from bcwaldon/configurable-channel

feat(channel): Allow user to configure CoreOS update channel
if exposing docker api locally use a unique port for each box in the cluster
Merge pull request #104 from ddollar/unique-port-forward

if exposing docker api locally use a unique port for each box in the cluster
Merge pull request #115 from shopkeep/allow-docker-host-vm

feat(Vagrantfile): alter configuration to allow use as Docker host vm
Fix up a couple of tyops.
Trivial indent, space v. tabs
Merge pull request #119 from thorduri/minor

Minor fix ups.
alpha is a string, and needs quoting
Merge pull request #120 from spkane/fix-config

alpha is a string, and needs quoting
Merge branch 'master' of https://github.com/coreos/coreos-vagrant

Conflicts:
	README.md
	Vagrantfile
	config.rb.sample