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_test.go

@@ -0,0 +1,78 @@
+package tmpl
+
+import (
+	"errors"
+	"github.com/golang/mock/gomock"
+	"gotest.tools/assert"
+	"testing"
+)
+
+func Test_fetchSecretValue(t *testing.T) {
+	controller := gomock.NewController(t)
+	defer controller.Finish()
+	c := NewMockvalClient(controller)
+	secretsClient = c
+
+	secretPath := "ref+vault://key/#path"
+	expectArg := make(map[string]interface{})
+	expectArg["key"] = secretPath
+
+	valsResult := make(map[string]interface{})
+	valsResult["key"] = "key_value"
+	c.EXPECT().Eval(expectArg).Return(valsResult, nil)
+	result, err := fetchSecretValue(secretPath)
+	assert.NilError(t, err)
+	assert.Equal(t, result, "key_value")
+}
+
+func Test_fetchSecretValue_error(t *testing.T) {
+	controller := gomock.NewController(t)
+	defer controller.Finish()
+	c := NewMockvalClient(controller)
+	secretsClient = c
+
+	secretPath := "ref+vault://key/#path"
+	expectArg := make(map[string]interface{})
+	expectArg["key"] = secretPath
+
+	expectedErr := errors.New("some error")
+	c.EXPECT().Eval(expectArg).Return(nil, expectedErr)
+	result, err := fetchSecretValue(secretPath)
+	assert.Equal(t, err, expectedErr)
+	assert.Equal(t, result, "")
+}
+
+func Test_fetchSecretValue_no_key(t *testing.T) {
+	controller := gomock.NewController(t)
+	defer controller.Finish()
+	c := NewMockvalClient(controller)
+	secretsClient = c
+
+	secretPath := "ref+vault://key/#path"
+	expectArg := make(map[string]interface{})
+	expectArg["key"] = secretPath
+
+	valsResult := make(map[string]interface{})
+	c.EXPECT().Eval(expectArg).Return(valsResult, nil)
+	result, err := fetchSecretValue(secretPath)
+	assert.Error(t, err, "unexpected error occurred, map[] doesn't have 'key' key")
+	assert.Equal(t, result, "")
+}
+
+func Test_fetchSecretValue_invalid_type(t *testing.T) {
+	controller := gomock.NewController(t)
+	defer controller.Finish()
+	c := NewMockvalClient(controller)
+	secretsClient = c
+
+	secretPath := "ref+vault://key/#path"
+	expectArg := make(map[string]interface{})
+	expectArg["key"] = secretPath
+
+	valsResult := make(map[string]interface{})
+	valsResult["key"] = 10
+	c.EXPECT().Eval(expectArg).Return(valsResult, nil)
+	result, err := fetchSecretValue(secretPath)
+	assert.Error(t, err, "expected 10 to be string")
+	assert.Equal(t, result, "")
+}