Distinct Property supports arbitrary limit

This PR enhances the distinct_property constraint such that a limit can
be specified in the RTarget/value parameter. This allows constraints
such as:

```
constraint {
  distinct_property = "${meta.rack}"
  value = "2"
}
```

This restricts any given rack from running more than 2 allocations from
the task group.

Fixes https://github.com/hashicorp/nomad/issues/1146

helper/funcs.go

@@ -76,6 +76,20 @@ func IntMin(a, b int) int {
 	return b
 }
 
+func IntMax(a, b int) int {
+	if a > b {
+		return a
+	}
+	return b
+}
+
+func Uint64Max(a, b uint64) uint64 {
+	if a > b {
+		return a
+	}
+	return b
+}
+
 // MapStringStringSliceValueSet returns the set of values in a map[string][]string
 func MapStringStringSliceValueSet(m map[string][]string) []string {
 	set := make(map[string]struct{})