fix(plugin): monkey-patch coreos plugin

Override the behavior of the upstream coreos guest
plugin to make it work with usr images.
doc(README): drop alt-setup
feat(provision): Add coreos-cloudinit provisioner
Merge pull request #45 from bcwaldon/fix-usr

Override upstream plugin; Add cloudinit provisioner
doc(README): Adjust to new command syntax for vagrant box remove
Merge pull request #48 from gmacario/fix-boxremove

doc(README): Adjust to new command syntax for vagrant box remove
fix(clustering): Use same codepath for single- and multi-node
Merge pull request #47 from bcwaldon/fix-usr

Implement clustering with coreos-cloudinit