Add nfs mount option to puppet provisioner

This mimics the equivalent feature from the chef_solo provisioner, and
mounts the puppet manifests and modules with NFS. Doing so can greatly
shortens the time of a puppet run if you have many .pp files.

Enabling this is optional. Virtualbox's (or any other provider's) shared
folders method stays the default. A typical usage would look like this:

    config.vm.provision :puppet do |puppet|
        puppet.manifests_path = "puppetmaster/manifests"
        puppet.module_path = ["puppetmaster/modules"]
        puppet.manifest_file  = "site.pp"
        puppet.nfs = true
    end

This fixes #1308.
update CHANGELOG