Allow passing VMs & disks folders as parameters

This patch allows the user to pass the folders for the virtual machines configuration files and virtual disks.

Example:

``` 
class { 'hyper_v':
  ensure_powershell         => present,
  ensure_tools              => present,
  virtual_hard_disks_folder => 'D:\',
  virtual_machines_folder   => 'D:\Hyper-V',
}
```

The patch is added because because OpenStack is able to handle with ```instance_path``` where to put the virtual disk, but we also want to configure where the configuration files are stored.

Take a look and let me know if it's good for you.