Abstract the get_machines part. Prepare for include offline.
Added new include_offline configuration item
Support include_offline boxes.

If include_offline is enabled, we loop over all available boxes picking
those with the correct provider.

If a box has no private ip set or is offline, it is skipped (a warning is shown).
The lambda that gets the ip will return nil for the boxes.
Merge pull request #14 from vStone/feature/include_offline_boxes

Added option to include offline boxes in the hosts file.