`local_state_tree` should not be mandatory in the Salt masterless provisioner

From my understanding of packer, Provisioner can be chain, the states folder could be already there and could come from :  
  
  - the shell provisioner which could download the states from somewhere
  - there is a Directories Provisionner on the way #251

Then the states folders could be already present and so shouldn't be a mandatory parameter. 

https://github.com/mitchellh/packer/blob/da4a7c3b7014b85fc319cd71f71bda9a6f3e6e2a/provisioner/salt-masterless/provisioner.go#L100-L104