loadbalance: fix {min,max}_nodes init and validation

The default kwargs for SGELoadBalancer's init were defaulting to
min_nodes=1, and max_nodes=None which always lead to an exception being
thrown about min_nodes being greater than max_nodes (1 > None == True).
Changed the default for min_nodes kwarg to None and moved
{min,max}_nodes validation to SGELoadBalancer.run().