Store the final `provision_enabled` value to env

Since 1.3.0 Vagrant uses a sentinel file to decide if it should run the provisioners or not (unless the user explicitly passes `--[no-]provision`). Store that knowledge back to the env to let other middleware actions to use it.

Now for example  vagrant-berkshelf does the cookbook dependency resolving and installing on every `vagrant up/reload` unless `--no-provision` is specified ([code](https://github.com/RiotGames/vagrant-berkshelf/blob/v1.3.4/lib/berkshelf/vagrant/env_helpers.rb#L60)).