[Libosinfo] Installer for Fedora netiso/F21

[Zeeshan Ali (Khattak)]

This series of patches make installer scripts working for Fedora 21 netiso. Unfortunately this couldn't have been handled (nicely) without addition of new API. So apps will have to change code to fix installer for Fedora 21. Things still yet to be addressed:

* Install scripts are still only associated with os entries but installer scripts don't work for all medias/variants. This is not a new problem though. For example our Fedora script was never working for netiso but we still advertised it as such (or at least we didn't inform apps in any way about this).

  I think we'll need API to optionally associate installer to variants to fix this issue.

* Netiso requires internet and we don't have any API to inform app about this. We already have API to inform apps that an installer (always) require internet but not if its the media that requires it.

  The solution here would simply be to add 'needs-internet' param/api to media entries IMO.