m/cli/metroctl: factor out some helper functions

We will need these to create metroctl-compatible configs from
//metropolis/cluster/launch.

Change-Id: I2705afefb62b7e1b35c87d9753c4ca9c7f534c26
Reviewed-on: https://review.monogon.dev/c/monogon/+/1324
Tested-by: Jenkins CI
Reviewed-by: Mateusz Zalega <mateusz@monogon.tech>
diff --git a/metropolis/cli/metroctl/k8scredplugin.go b/metropolis/cli/metroctl/k8scredplugin.go
index e434a61..605bc87 100644
--- a/metropolis/cli/metroctl/k8scredplugin.go
+++ b/metropolis/cli/metroctl/k8scredplugin.go
@@ -10,6 +10,8 @@
 	"github.com/spf13/cobra"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	clientauthentication "k8s.io/client-go/pkg/apis/clientauthentication/v1"
+
+	"source.monogon.dev/metropolis/cli/metroctl/core"
 )
 
 var k8scredpluginCmd = &cobra.Command{
@@ -23,8 +25,8 @@
 }
 
 func doK8sCredPlugin(cmd *cobra.Command, args []string) {
-	cert, key, err := getCredentials()
-	if err == noCredentialsError {
+	cert, key, err := core.GetOwnerCredentials(flags.configPath)
+	if err == core.NoCredentialsError {
 		log.Fatal("No credentials found on your machine")
 	}
 	if err != nil {