Don't consider stderr CLI output to be an error

Between RabbitMQ v3.8.14 and v3.8.15, the command
`rabbitmq-queues rebalance quorum` outputs warn messages like
`10:59:50.674 [warn]  Node :"[email protected]" only
contains 1 queues, do nothing` to stderr.

Although this CLI command exits with 0, the operator considers this as a
failure, retrying to execute that command. The operator logs the
following:
`2021-03-19T08:39:56.305Z	ERROR
controller-runtime.manager.controller.rabbitmqcluster	Reconciler error
{"reconciler group": "rabbitmq.com", "reconciler kind":
  "RabbitmqCluster", "name": "r1", "namespace": "default", "error":
    "failed to run queue rebalance on pod r1-server-0:`

In general when CLI commands output to stderr, it doesn't mean that
there was an error. So, in this commit we only consider non-zero exit
codes as error.

config/crd/bases/rabbitmq.com_rabbitmqclusters.yaml

@@ -3499,7 +3499,7 @@ spec:
                     additionalPlugins:
                       description: 'List of plugins to enable in addition to essential plugins: rabbitmq_management, rabbitmq_prometheus, and rabbitmq_peer_discovery_k8s.'
                       items:
-                        description: kubebuilder validating tags 'Pattern' and 'MaxLength' must be specified on string type. Alias type 'string' as 'Plugin' to specify schema validation on items of the list 'AdditionalPlugins'
+                        description: A Plugin to enable on the RabbitmqCluster.
                         maxLength: 100
                         pattern: ^\w+$
                         type: string

controllers/pod_executor.go

@@ -3,7 +3,7 @@ package controllers
 import (
 	"bufio"
 	"bytes"
-	"fmt"
+
 	corev1 "k8s.io/api/core/v1"
 	"k8s.io/client-go/kubernetes"
 	"k8s.io/client-go/kubernetes/scheme"
@@ -48,12 +48,6 @@ func (p *rabbitmqPodExecutor) Exec(clientset *kubernetes.Clientset, clusterConfi
 		Stdin:  nil,
 		Tty:    false,
 	})
-	if err != nil {
-		return stdOut.String(), stdErr.String(), err
-	}
-	if stdErr.Len() > 0 {
-		return stdOut.String(), stdErr.String(), fmt.Errorf("%v", stdErr)
-	}
 
-	return stdOut.String(), "", nil
+	return stdOut.String(), stdErr.String(), err
 }