Add the ability to disable NFS in vagrant

Using the Vagrant 1.6.5. Docker provider with VMware Fusion, seems to have issues. There is a bug against Vagrant for this (since 1.6.3) here: https://github.com/mitchellh/vagrant/issues/4011

In the meantime, it looks like this additional configuration allows one to work around it, by forcefully disabling NFS, so that vagrant falls back to rsync.