Cluster: redis-trib cluster allocation more even across nodes.

redis-trib used to allocate slots not considering fractions of nodes
when computing the slots_per_node amount. So the fractional part was
carried over till the end of the allocation, where the last node
received a few more slots than any other (or a lot more if the cluster
was composed of many nodes).

The computation was changed to allocate slots more evenly when they are
not exactly divisible for the number of masters we have.

src/redis-trib.rb

@@ -510,7 +510,6 @@ class RedisTrib
     def alloc_slots
         nodes_count = @nodes.length
         masters_count = @nodes.length / (@replicas+1)
-        slots_per_node = ClusterHashSlots / masters_count
         masters = []
         slaves = []
 
@@ -541,13 +540,18 @@ class RedisTrib
         end
 
         # Alloc slots on masters
-        i = 0
+        slots_per_node = ClusterHashSlots.to_f / masters_count
+        first = 0
+        cursor = 0.0
         masters.each_with_index{|n,masternum|
-            first = i*slots_per_node
-            last = first+slots_per_node-1
-            last = ClusterHashSlots-1 if masternum == masters.length-1
+            last = (cursor+slots_per_node-1).round
+            if last > ClusterHashSlots || masternum == masters.length-1
+                last = ClusterHashSlots-1
+            end
+            last = first if last < first # Min step is 1.
             n.add_slots first..last
-            i += 1
+            first = last+1
+            cursor += slots_per_node
         }
 
         # Select N replicas for every master.