Proxmox support for Salt-Cloud

Pfew, this one was actually a bit trickier than I had hoped it would be but I've done it :-)

Since the Proxmox REST API works with ids rather than names that had to be solved as well although it does require a little more communication with the Proxmox API in order to get the required information. It should be pretty optimal the way it is now, an option would be caching some information but that would also result in potential race conditions when there are multiple provision tasks (either via Salt-cloud, the CLI, API or webinterface). I'd prefer to choose reliability over speed in this matter.

There are a few caveats:
While Proxmox does support both KVM and OpenVZ only the latter is fully integrated with automatic provisioning in Salt-cloud since KVM requires the user to manually install the OS. 
KVM machines can be launched, but no provisioning can be done at this time.

When using OpenVZ with the default (and recommended) settings (that is: using venet adapters instead of veth) it **cannot** work with DHCP or automatically provisioned IP's. The user is required to provide an IP when creating the vm. The IP is required in order to start the VM and is also being used to SSH to the machine and install the required role(s). 

Documentation is included, so this should be good to go I guess?