OS ISO file names should be overridable

[obilodeau]

See @Svieg's comment in #5.

If failed file:// references are allowed in iso_urls array I think this is reasonable. We provide default values in config-examples.json and allow user to override. If a user doesn't provide a valid iso_path and/or iso_filename then it would fallback to download URL (in the case that evaluation ISOs are available like for Windows 10).

So it would look like this from the user's perspective:

{
  "iso_path": "/home/olivier/gosecure/tools/",
  "iso_win7_x86": "win7_x86.iso"
...