Allow load balancer to accommodate different region server configurations

Robert Dyer raised the following scenario under the thread of 'Multiple regionservers on a single node':

I have a very small cluster where all nodes are identical. However, I was
just given a very powerful node to add into this cluster which effectively
doubles the total CPUs, RAM, and HDDs in the cluster.

As such, when I run a MR job half the jobs go to this single, new node yet
most of the data is not local due to HBase balancing the regions.

Load balancer should take region server config (total heap in the above case) into account when allocating regions.