feat(template): added secret template function (#1221)

* feat(tmpl): added fetchSecretValue template function

This adds a tmpl `fetchSecretValue` and `expandSecretRefs` function by:
- Adding:
    - `expandSecretRefs` function in tmpl package that uses vals
    package to fetch secrets
    - `fetchSecretValue` function in tmpl package like below but for
    single string value
    - gomock for tests purpose
- Changing:
    - move init of vals package to function (so the same instance can be used for template values and rendering the whole template)

* doc(secret): added doc how to use new tmpl methods

Added example usage of `fetchSecretValue` and `expandSecretRefs`

pkg/app/app.go

@@ -16,17 +16,13 @@ import (
 
 	"github.com/roboll/helmfile/pkg/argparser"
 	"github.com/roboll/helmfile/pkg/helmexec"
+	"github.com/roboll/helmfile/pkg/plugins"
 	"github.com/roboll/helmfile/pkg/remote"
 	"github.com/roboll/helmfile/pkg/state"
 	"github.com/variantdev/vals"
 	"go.uber.org/zap"
 )
 
-const (
-	// cache size for improving performance of ref+.* secrets rendering
-	valsCacheSize = 512
-)
-
 type App struct {
 	OverrideKubeContext string
 	OverrideHelmBinary  string
@@ -95,7 +91,7 @@ func Init(app *App) *App {
 	app.directoryExistsAt = directoryExistsAt
 
 	var err error
-	app.valsRuntime, err = vals.New(vals.Options{CacheSize: valsCacheSize})
+	app.valsRuntime, err = plugins.ValsInstance()
 	if err != nil {
 		panic(fmt.Sprintf("Failed to initialize vals runtime: %v", err))
 	}