Use builtin synced folders if available

I'm proposing this solution for using core synced folder plugins without dropping the legacy rsync code for people that are still on Vagrant < 1.4.

In my opinion such a solution could help fixing many issues related to rsync and could also make [Windows support](https://github.com/mitchellh/vagrant-rackspace/pull/93) easier to achieve.

Please note that I've tested the modification only on Vagrant 1.6

Thanks in advance for your attention.