core: add Vagrantfile class, tests
core: Environment uses the new Vagrantflie class
core: Environment#config_global is gone!
core: Vagrantfile class now adheres to box_version
core: get rid of all uses of config_global
core: plugin tests no longer obliterate manager
core: Vagrantfile#machine_config returns more info now
core: Vagrantfile can create machines, Environment#machine uses that
core: update docs for Vagrant::Environment
core: Alphabetize functions and comment better in Environment
core: Machine has reference to Vagrantfile
core: fix Machine tests
Merge pull request #2947 from mitchellh/f-environment-refactor

Core refactor: "Vagrantfile" abstraction