Make sure kanctl honors KUBECONFIG env and kubeconfig dir (#1247)

* Make sure `kanctl` honors `KUBECONFIG` env and kubeconfig dir

kanctl in current implentation create kubeclient using `inClusterConfig`
it doesn't check if the `KUBECONFIG` env var is set or config file is
available at default location.
This has resulted into problem for some of the users. This PR makes the
change to prioritise creating kubeclient in below order

- KUBECONFIG env var
- kubeconfig file at deafult location `~/.kube/config`
- Use `inClusterConfig`

* Address review comments

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

pkg/kube/client.go

@@ -15,6 +15,8 @@
 package kube
 
 import (
+	"os"
+
 	"github.com/pkg/errors"
 	crdclient "k8s.io/apiextensions-apiserver/pkg/client/clientset/clientset"
 	"k8s.io/client-go/dynamic"
@@ -46,9 +48,19 @@ func ConfigNamespace() (string, error) {
 
 // LoadConfig returns a kubernetes client config based on global settings.
 func LoadConfig() (*rest.Config, error) {
+	kubeConfigEnv := os.Getenv(clientcmd.RecommendedConfigPathEnvVar)
+	if len(kubeConfigEnv) != 0 {
+		return clientcmd.BuildConfigFromFlags("", kubeConfigEnv)
+	}
+
+	if c, err := clientcmd.BuildConfigFromFlags("", clientcmd.RecommendedHomeFile); err == nil {
+		return c, nil
+	}
+
 	if c, err := rest.InClusterConfig(); err == nil {
 		return c, nil
 	}
+
 	return newClientConfig().ClientConfig()
 }