Added support for datacenters in subfolders

As mentioned here: https://github.com/ezrapagel/knife-vsphere/pull/69
Datacenters in subfolders cannot be found. I've patched this to traverse all folders looking for the specified datacenter.

Caveat: If you previously specified your datacenter in knife.rb as something like "North America/Texas DC", you should now only use "Texas DC". Obviously this won't work if you have two datacenters with the same name in different folders, but I'm not sure if VMWare even allows that.