support for multiple portstrees

As I was working on multiple portstrees to build custom ports, I figured I should add support for it in puppet-poudriere. I kept some backward compatibility in init.pp but that could be removed later on.
While on it, I added 'ensure' parameters to both poudriere::env and poudriere::portstree so they can be removed later on.
Minor style thing, but I'd prefer to keep dependencies as the last param in the resource.  So just require => to the bottom here please.