core: MachineIndex stores the box associated with a machine when ID is set
core: Store the actual box data, not just the name
core: MachineIndex::Entry#valid? method and tests
core: Environment#action_runner sets machine_index
commands/box/remove: if box is in use, warn user, ask for confirmation
commands/box/remove: add --force flag
commands/global-status: call proper method
Fix some tests
core: environment_loaded doesn't load full env
Merge pull request #3538 from mitchellh/f-box-usage

Track box usage (local) to know if envs are using boxes