Allow Libvirt provisioner custom settings

[br0ziliy]

This is WIP for RFE #121. Not (yet) tested, so please don't yet merge, I have couple of things I don't like in this diff, and want some 2nd opinion:

• argv processing. Can't we finally rewrite command line arguments block to use getopt?
• assignments on line 1519 and further - can this be simplified somehow, or it's fine as it is?

[purpleidea]

@br0ziliy Sweet... Okay some replies:

1. Re: argv processing, we should definitely "libify" this and use getopt or whatever ruby thing is best if it will work. So that should be a separate issue (and even a pre-req to this one) and I'll happily accept that patch. Your patch as is, I like though!

Small bug: in the parser add on, I think it won't work correctly for the connect_via_ssh key, since that's a bool, not a string, and your parse always assumes string values.

2. Re: line 1519: I think it looks fine, but maybe there is a ruby idiom I don't know about. Better to ask someone else, otherwise I'm fine to merge that part.

3. One small nitpick: can you move the :libvirt key one up so it's above the comment field, and so comment is always last in the omv.yaml file? I think it will be a bit cleaner this way. Remember that this patch should also include empty :libvirt keys to all the existing example files please :)

Obviously doc patches are appreciated, but I wouldn't block the merge without them.
It looks amazing! 10/10, would review again. Thanks!

[purpleidea]

@br0ziliy ping, want to update/rebase this lovely patch so we can merge it?

Thanks!