Boot script improvements
Boot script improvements

- Document env vars and functions
- Define which things are public/private
- Move pre_start events in to places where they
  can do more prior to startup
- Add RELEASE_CONFIG_DIR env var, for defining
  where to look for vm.args, sys.config, and other
  config related files. By default it will check first
  in the release root directory, and then the release version
  directory.

bitwalker/distillery,jsierles/distillery,bitwalker/distillery,jsierles/distillery