Register GCP auth plugin

As described here hashicorp/terraform#15244 (comment)
client-go doesn't enable GCP auth plugin by default. We have to do it explicitly

Author: e-max

kubernetes/provider.go

@@ -9,6 +9,7 @@ import (
 	"github.com/hashicorp/terraform/helper/schema"
 	"github.com/hashicorp/terraform/terraform"
 	"github.com/mitchellh/go-homedir"
+	_ "k8s.io/client-go/plugin/pkg/client/auth/gcp"
 	restclient "k8s.io/client-go/rest"
 	"k8s.io/client-go/tools/clientcmd"
 	clientcmdapi "k8s.io/client-go/tools/clientcmd/api"