Converge draining

Implementing the logic for https://github.com/rackerlabs/otter/issues/715

Connections aren't a stat we can get yet, so connections will always be None for now (and so will never == 0).  Just wanted to add this logic so that when we do get the stats it's easy.
Oh cool, thanks!  I was worried that there would be other statuses like 'REBUILD' or something, but forgot they were no longer strings straight from Nova.  Will do so, thanks!