kubelet: Add description for failed predicate Events

**Problem:** When a Pod fails to run due to kubelet-side admission checks, the
v1.Event objects emitted looks like these:

    Reason="NodeAffinity" Message="Predicate NodeAffinity failed"
    Reason="NodeName" Message="Predicate NodeName failed"
    Reason="NodePorts" Message="Predicate NodePorts failed"

**Solution:** Expose human-readable predicate description in the failure
so that kubelet submits an Event with this description, such as:

    Reason="NodeAffinity" Message="Predicate NodeAffinity failed: node(s) didn't match Pod's node affinity/selector"
    Reason="NodeName" Message="Predicate NodeName failed: node(s) didn't match the requested node name"
    Reason="NodePorts" Message="Predicate NodePorts failed: node(s) didn't have free ports for the requested pod ports"

Author: ahmetb

pkg/kubelet/lifecycle/predicate.go

@@ -269,7 +269,7 @@ type PredicateFailureError struct {
 }
 
 func (e *PredicateFailureError) Error() string {
-	return fmt.Sprintf("Predicate %s failed", e.PredicateName)
+	return fmt.Sprintf("Predicate %s failed: %s", e.PredicateName, e.PredicateDesc)
 }
 
 // GetReason returns the reason of the PredicateFailureError.