add a proxmox inventory plugin

I'd like to add a dynamic inventory plugin for Proxmox (http://www.proxmox.com).

There is a closed issue for this feature (#2784).

The code is actualy not well tested and probably too soon for any merge. For example, I like to add useful variables in the inventory.

Any tests, comments or wish are welcome (even tips to make the code clean enough to be merged) !