Skip disabled shared folders

Similar to the [virtualbox provider](https://github.com/mitchellh/vagrant/blob/master/plugins/providers/virtualbox/action/share_folders.rb#L40-L41), this pull request skips any disabled `synced_folder` definitions instead of using `rsync` on them.

It allows you to disable the `/vagrant` directory being rsynced to the joyent server.

```ruby
Vagrant.configure('2') do |config|
  config.vm.box = 'dummy'

  config.vm.provider :joyent do |joyent|
    # ...
  end

  # Don't rsync the project to the server
  config.vm.synced_folder '.', '/vagrant', disabled: true
end
```