Enhance the synced folder plugin implementation with a cleanup routine

I propose adding an additional method to the synced folder plugin implementation, mainly for doing cleanup on the host after destroying a machine.

Something like this stub:
https://github.com/emyl/vagrant/commit/6548a6863f66d7e9a834fdb4ffac010b495e4f51

If you'd like to point me in the right direction I'll be happy to implement and open a PR.