Add --leader-election-namespace flag (#123)

Fixes (part of) aws-controllers-k8s/community#1753

This patch introduces a new `--leader-election-namespace` flag used to
configure controller-runtime leaderElection componenet. This namespace
is utilized by the controller to manage the `coordination.k8s.io/lease`
obejct for leader election.

In the context of the controller-runtime library, if the
LeaderElectionNamespace parametere is not explicitly set, the library
will automatically default its value to the content of the file
mounted at /var/run/secrets/kubernetes.io/serviceaccount/namespace.

> https://github.com/kubernetes-sigs/controller-runtime/blob/main/pkg/leaderelection/leader_election.go#L112-L127

In Kubernetes, when a pod is created, a service account is
automatically associated with it, unless explicitly specified
otherwise. This service account contains relevant information, such
as the namespace in which the pod is deployed. The Kubernetes API
server mounts a two files for the service account in the pod's
filesystem at /var/run/secrets/kubernetes.io/serviceaccount/token
and /var/run/secrets/kubernetes.io/serviceaccount/namespace,
respectively.

> https://github.com/kubernetes/kubernetes/blob/master/pkg/controller/serviceaccount/tokens_controller.go#L399-L402

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

pkg/config/config.go

@@ -38,6 +38,7 @@ import (
 
 const (
 	flagEnableLeaderElection           = "enable-leader-election"
+	flagLeaderElectionNamespace        = "leader-election-namespace"
 	flagMetricAddr                     = "metrics-addr"
 	flagEnableDevLogging               = "enable-development-logging"
 	flagAWSRegion                      = "aws-region"
@@ -72,6 +73,7 @@ var (
 type Config struct {
 	MetricsAddr                    string
 	EnableLeaderElection           bool
+	LeaderElectionNamespace        string
 	EnableDevelopmentLogging       bool
 	AccountID                      string
 	Region                         string
@@ -111,6 +113,23 @@ func (cfg *Config) BindFlags() {
 		"Enable leader election for controller manager. "+
 			"Enabling this will ensure there is only one active controller manager.",
 	)
+	flag.StringVar(
+		// In the context of the controller-runtime library, if the LeaderElectionNamespace parametere is not
+		//  explicitly set, the library will automatically default its value to the content of the file
+		// mounted at /var/run/secrets/kubernetes.io/serviceaccount/namespace.
+		// https://github.com/kubernetes-sigs/controller-runtime/blob/main/pkg/leaderelection/leader_election.go#L112-L127
+		//
+		// In Kubernetes, when a pod is created, a service account is automatically associated with it,
+		// unless explicitly specified otherwise. This service account contains relevant information, such
+		// as the namespace in which the pod is deployed. The Kubernetes API server mounts a two files
+		// for the service account in the pod's filesystem at /var/run/secrets/kubernetes.io/serviceaccount/token
+		// and /var/run/secrets/kubernetes.io/serviceaccount/namespace, respectively.
+		// https://github.com/kubernetes/kubernetes/blob/master/pkg/controller/serviceaccount/tokens_controller.go#L399-L402
+		&cfg.LeaderElectionNamespace, flagLeaderElectionNamespace,
+		"",
+		"Specific namespace that the controller will utilize to manage the coordination.k8s.io/lease object for leader election."+
+			" By default it will try to use the namespace of the service account mounted to the controller pod.",
+	)
 	flag.BoolVar(
 		&cfg.EnableDevelopmentLogging, flagEnableDevLogging,
 		false,