BUG: naive attempt at fixing #2970.

This adds a config_dir settings to the salt provisioner, with a 'sensible' default to /etc or C:\\etc on windows. This is then used to upload the minion file at the right place.

TBH, I think the current configuration setup of the vagrant salt plugin is a bit clumsy. What about adding this instead:

```
# Vagrantfile
 config.vm.provision :salt do |salt|

      salt.masterless = true # default false ?
      salt.minion_id = 'something' # default is using salt default

 end
```

This way, people don't need to care about how to actually set up masterless at the minion level, and translating this config into actual salt call is simple (e.g. salt-call ... --local --id=$id). Thoughts ? Would be happy to prepare a PR toward this if this makes sense to vagrant devs.
`.nil?` is preferred over `== nil`