Use binary releases for cluster push scripts.

This is for GCE right now.  Other clouds/clusters are probably broken.
Tweak salt docs a little
Vagrant now using pre-built binaries.
Fix up kubecfg.sh script to find correct binary.
Remove unused manifest file
GCS uploads as part of release are now marked public and non-cached by default.
Normalize some of the boilerplate in each of the `build` commands.
Update build/README.md