Added error handler for empty runtime entry

If there is no container runtime defined in the airshipctl
config then the error message is not clear. New handler shows
specific error description

Change-Id: Ib5b735ddaa1556a8a88fdb15c4b3942cd010db9b

pkg/container/container.go

@@ -37,6 +37,8 @@ type Container interface {
 //   * docker
 func NewContainer(ctx *context.Context, driver string, url string) (Container, error) {
 	switch driver {
+	case "":
+		return nil, ErrNoContainerDriver{}
 	case "docker":
 		cli, err := NewDockerClient(ctx)
 		if err != nil {

pkg/container/container_test.go

@@ -16,20 +16,25 @@ package container
 
 import (
 	"context"
-	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 )
 
 func TestNewContainer(t *testing.T) {
+	assert := assert.New(t)
+
 	ctx := context.Background()
-	_, actualErr := NewContainer(&ctx, "test_drv", "")
-	expectedErr := ErrContainerDrvNotSupported{Driver: "test_drv"}
-	errS := fmt.Sprintf(
-		"Call NewContainer should have returned error %s, got %s",
-		expectedErr,
-		actualErr,
-	)
-	assert.Equal(t, actualErr, expectedErr, errS)
+
+	t.Run("not-supported-container", func(t *testing.T) {
+		cnt, err := NewContainer(&ctx, "test_drv", "")
+		assert.Equal(nil, cnt)
+		assert.Equal(ErrContainerDrvNotSupported{Driver: "test_drv"}, err)
+	})
+
+	t.Run("empty-container", func(t *testing.T) {
+		cnt, err := NewContainer(&ctx, "", "")
+		assert.Equal(nil, cnt)
+		assert.Equal(ErrNoContainerDriver{}, err)
+	})
 }

pkg/container/errors.go

@@ -44,3 +44,11 @@ type ErrContainerDrvNotSupported struct {
 func (e ErrContainerDrvNotSupported) Error() string {
 	return fmt.Sprintf("Driver %s is not supported", e.Driver)
 }
+
+// ErrNoContainerDriver returned if no runtime defined in config
+type ErrNoContainerDriver struct {
+}
+
+func (e ErrNoContainerDriver) Error() string {
+	return fmt.Sprintf("container runtime is not defined in airshipctl config")
+}