Add sync_folders action support

Vagrant's docker provider calls sync_folders action to sync folders with the host machine and expose them as a volume in docker containers (https://github.com/mitchellh/vagrant/blob/master/plugins/providers/docker/action/host_machine_sync_folders.rb). Usually VM providers do not implement sync_folders action and docker provider uses Vagrant::Action::Builtin::SyncedFolders as a fallback. But this implementation does not work with NFS synced folders because they require additional environment configuration.
There is a workaround (https://github.com/YungSang/vagrant-docker-helper). But i would prefer to do not duplicate code but extend the host VM provider.
Add sync_folders action support

Check env for presence of nfs folder in sync_folder_config.synced_folders. If there  is no configured nfs folders for the machine nor in this hash, we will skip extra work to populate nfs related fields in the environment.
Merge pull request #142 from ilucker/fix/sync_folders

Add sync_folders action support

Related to mitchellh/vagrant#4011