Hook before the `Provision` to get control later in the middleware stack

To allow [vagrant-proxyconf](https://github.com/tmatilai/vagrant-proxyconf) plugin to configure environment for us, we hook before instead of after the `Provision` actions. This way we get back the control [in our action class](https://github.com/schisamo/vagrant-omnibus/blob/v1.1.0/lib/vagrant-omnibus/action/install_chef.rb#L35) later.

This helps tmatilai/vagrant-proxyconf#13.