Support hooking around provisioners runs

[fgrehm]

This PR implements GH-2044 and although I wasn't able to spike on changing vagrant-cachier code to remove the monkey patches we have in place, I wanted to open this up for discussion / code review as I was able to spot 2 other use cases for it :)

Apart from using it on vagrant-cachier, I want to use this feature to finally support fgrehm/vagrant-notify#5 and another use case would be a plugin that keeps track of the time taken on each provisioner run and reports back to the user, something like vagrant-aws's TimedProvision but targeting multiple providers

To try this out, you can use the following Vagrantfile using this branch:

class Action
  def initialize(app, env)
    @app = app
  end

  def call(env)
    puts 'before'
    puts "name: #{env[:provider_name]}"
    @app.call env
    puts 'after'
  end
end

class Plugin < Vagrant.plugin('2')
  name 'testing'
  action_hook 'hook-name', :provisioner_run do |hook|
    hook.before Vagrant::Action::Builtin::ProvisionerRun, Action
  end
end

Vagrant.configure("2") do |config|
  config.vm.box = "some-box"
  config.vm.provision :shell, inline: 'echo hello'
  config.vm.provision :shell, inline: 'echo hello 2'
end

[mitchellh]

This is a great idea. I'm going to modify it slightly but merge this. I'll CC You on the modification, and might just blow your mind.