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/tmpl/expand_secret_ref.go

@@ -0,0 +1,63 @@
+package tmpl
+
+import (
+	"errors"
+	"fmt"
+	"github.com/roboll/helmfile/pkg/plugins"
+	"github.com/variantdev/vals"
+	"sync"
+)
+
+//to generate mock run mockgen -source=expand_secret_ref.go -destination=expand_secrets_mock.go -package=tmpl
+type valClient interface {
+	Eval(template map[string]interface{}) (map[string]interface{}, error)
+}
+
+var once sync.Once
+var secretsClient valClient
+
+func fetchSecretValue(path string) (string, error) {
+	tmpMap := make(map[string]interface{})
+	tmpMap["key"] = path
+	resultMap, err := fetchSecretValues(tmpMap)
+	if err != nil {
+		return "", err
+	}
+
+	rendered, ok := resultMap["key"]
+	if !ok {
+		return "", errors.New(fmt.Sprintf("unexpected error occurred, %v doesn't have 'key' key", resultMap))
+	}
+
+	result, ok := rendered.(string)
+	if !ok {
+		return "", errors.New(fmt.Sprintf("expected %v to be string", rendered))
+	}
+
+	return result, nil
+}
+
+func fetchSecretValues(values map[string]interface{}) (map[string]interface{}, error) {
+	var err error
+	// below lines are for tests
+	once.Do(func() {
+		var valRuntime *vals.Runtime
+		if secretsClient == nil {
+			valRuntime, err = plugins.ValsInstance()
+			if err != nil {
+				return
+			}
+			secretsClient = valRuntime
+		}
+	})
+	if secretsClient == nil {
+		return nil, err
+	}
+
+	resultMap, err := secretsClient.Eval(values)
+	if err != nil {
+		return nil, err
+	}
+
+	return resultMap, nil
+}