command/validate: read terraform.tfvars file for variable values

This is now consistent with the handling of this file for other commands.

command/test-fixtures/validate-valid/with-tfvars-file/main.tf

@@ -0,0 +1,4 @@
+variable "var_without_default" {
+  type = "string"
+}
+

command/test-fixtures/validate-valid/with-tfvars-file/terraform.tfvars

@@ -0,0 +1 @@
+var_without_default = "foo"

command/validate.go

@@ -17,7 +17,7 @@ type ValidateCommand struct {
 const defaultPath = "."
 
 func (c *ValidateCommand) Run(args []string) int {
-	args, err := c.Meta.process(args, false)
+	args, err := c.Meta.process(args, true)
 	if err != nil {
 		return 1
 	}

command/validate_test.go

@@ -1,9 +1,11 @@
 package command
 
 import (
+	"os"
 	"strings"
 	"testing"
 
+	"github.com/hashicorp/terraform/helper/copy"
 	"github.com/mitchellh/cli"
 )
 
@@ -28,6 +30,28 @@ func TestValidateCommand(t *testing.T) {
 	}
 }
 
+func TestValidateCommandWithTfvarsFile(t *testing.T) {
+	// Create a temporary working directory that is empty because this test
+	// requires scanning the current working directory by validate command.
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("validate-valid/with-tfvars-file"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	ui := new(cli.MockUi)
+	c := &ValidateCommand{
+		Meta: Meta{
+			testingOverrides: metaOverridesForProvider(testProvider()),
+			Ui:               ui,
+		},
+	}
+
+	args := []string{}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad %d\n\n%s", code, ui.ErrorWriter.String())
+	}
+}
+
 func TestValidateFailingCommand(t *testing.T) {
 	if ui, code := setupTest("validate-invalid"); code != 1 {
 		t.Fatalf("Should have failed: %d\n\n%s", code, ui.ErrorWriter.String())