update CoreOS installation guides

* CoreOS guides now use the 0.5.4 official release binaries
* kube-register has been updated to work with the 0.5.4 release
* flannel uses the vxlan backend by default for better performance
* cloud-configs are used for all examples
* instructions for GCE, AWS, and VMware Fusion have been combined
Nit: Might be worthwhile to point to where to find the latest images for other regions/instance types, etc.