GCE: Allow nodes to exceed target pool maximums

If we would exceeded the TargetPool API maximums, instead just
randomly select some subsection of the nodes to include in the TP
instead.

pkg/util/rand/rand.go

@@ -65,3 +65,19 @@ func String(length int) string {
 	}
 	return string(b)
 }
+
+// A type that satisfies the rand.Shufflable interface can be shuffled
+// by Shuffle. Any sort.Interface will satisfy this interface.
+type Shufflable interface {
+	Len() int
+	Swap(i, j int)
+}
+
+func Shuffle(data Shufflable) {
+	rng.Lock()
+	defer rng.Unlock()
+	for i := 0; i < data.Len(); i++ {
+		j := rng.rand.Intn(i + 1)
+		data.Swap(i, j)
+	}
+}