ENH: simpler masterless configuration.

This PR simplifies vagrant-salt configuration for masterless setup: instead of having people messing with the configuration files (which does not work ATM because of #2940), people can simply do the following:

    config.vm.provision :salt do |salt|
      salt.masterless = true # false by default
      salt.minion_id = "base" # allows to override the id
  end