honor disabling of synced folders

With a Vagrantfile snippet like this

```ruby
config.vm.synced_folder ".", "/vagrant", disabled: true
```

the rsync'ing of the default folder can be disabled.
See also
https://github.com/mitchellh/vagrant-aws/commit/48d424a401045260973750f83719f43b7707c438
enable controlling multiple VMs in parallel

e.g. `vagrant up /swift/ --provider=openstack --parallel`
update README.md

to include `os.user_data` and some more comments