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`

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/Masterminds/sprig v2.22.0+incompatible
 	github.com/aryann/difflib v0.0.0-20170710044230-e206f873d14a
 	github.com/go-test/deep v1.0.3
+	github.com/golang/mock v1.4.3
 	github.com/google/go-cmp v0.4.0
 	github.com/gosuri/uitable v0.0.3
 	github.com/hashicorp/go-getter v1.3.0
@@ -29,9 +30,8 @@ require (
 	go.uber.org/zap v1.9.1
 	golang.org/x/crypto v0.0.0-20191029031824-8986dd9e96cf // indirect
 	golang.org/x/sync v0.0.0-20200317015054-43a5402ce75a
-	golang.org/x/time v0.0.0-20191024005414-555d28b269f0 // indirect
-	google.golang.org/appengine v1.6.5 // indirect
 	gopkg.in/square/go-jose.v2 v2.4.0 // indirect
 	gopkg.in/yaml.v2 v2.2.4
+	gotest.tools v2.2.0+incompatible
 	gotest.tools/v3 v3.0.3-0.20200410202438-4e4a41b7851a
 )