added synced_folder entries to Vagrantfile

Added two config.vm.synced_folder entries to enable a couple of useful
guest directories to be available from the host system.

This config does not create any directories on the host or guest system,
so before 'vagrant up' is run, the host directories (./sync/...) must be
created. Vagrant warns and exits if the local directories are not present.
See https://docs.vagrantup.com/v2/providers/configuration.html for syntax on overriding config in a provider - you need to use a second variable in the do block - as in ac72a395165dd6625a6e10174a82452fa010f632.

Also it looks like vagrant-aws __does__ [support synced folders](https://github.com/mitchellh/vagrant-aws/blob/master/README.md#synced-folders) - but it only syncs to the VM so it wouldn't help in this case. Let's put a comment about why we're configuring it here.