Balancer should avoid leaving idle region servers

After HBASE-15529 the StochasticLoadBalancer makes the decision to run based on its internal cost functions rather than the simple region count skew of BaseLoadBalancer.

Given the default weights for those cost functions, the default minimum cost to indicate a need to rebalance, and a regions per region server density of ~90 we are not very responsive to adding additional region servers for non-trivial cluster sizes:

• For clusters ~10 nodes, the defaults think a single RS at 0 regions means we need to balance
• For clusters >20 nodes, the defaults will not consider a single RS at 0 regions to mean we need to balance. 2 RS at 0 will cause it to balance.
• For clusters ~100 nodes, having 6 RS with no regions will still not meet the threshold to cause a balance.

Note that this is the decision to look at balancer plans at all. The calculation is severely dominated by the region count skew (it has weight 500 and all other weights are ~105), so barring a very significant change in all other cost functions this condition will persist indefinitely.

Two possible approaches:

• add a new cost function that's essentially "don't have RS with 0 regions" that an operator can tune
• add a short circuit condition for the needsBalance method that checks for empty RS similar to the check we do for colocated region replicas

For those currently hitting this an easy work around is to set hbase.master.balancer.stochastic.minCostNeedBalance to 0.01. This will mean that a single RS having 0 regions will cause the balancer to run for clusters of up to ~90 region servers. It's essentially the same as the default slop of 0.01 used by the BaseLoadBalancer.