Rename _vagrant to vagrant

Apparently the idea to prefix a package with an underscore is not that
smart as I thought. Yes `go test` does not run it by default when you
run `go test ./...` but also other commands like `go mod tidy` do not
work consistently.

Nothing changes in practice. By default only unit tests run. Setting the
new environment variable: `TEST_WITH_VAGRANT` you include the test who
uses vagrant.

Signed-off-by: Gianluca Arbezzano <gianarb92@gmail.com>

test/vagrant/vagrant_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"net/http"
+	"os"
 	"testing"
 	"time"
 
@@ -16,6 +17,11 @@ import (
 )
 
 func TestVagrantSetupGuide(t *testing.T) {
+	_, ok := os.LookupEnv("TEST_WITH_VAGRANT")
+	if !ok {
+		t.Skip("You can run e2e tests that depends from Vagrant setting the envvar TEST_WITH_VAGRANT")
+	}
+
 	ctx := context.Background()
 
 	machine, err := vagrant.Up(ctx,