Use gopkg.in/yaml.v2 for Helmfile v0.x (#609)

This should fix #435 for Helmfile v0.x releases since the next v0.150.0.
We introduce a new envvar to opt-in to the new YAML library, so that you can give it a shot before upgrading your Helmfile to v1. The same envvar can be used to opt-out of the new YAML library after you upgrade to Helmfile v1, giving you a more flexible migration story.

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

pkg/tmpl/context_funcs.go

@@ -17,6 +17,7 @@ import (
 
 	"github.com/helmfile/helmfile/pkg/envvar"
 	"github.com/helmfile/helmfile/pkg/helmexec"
+	"github.com/helmfile/helmfile/pkg/maputil"
 	"github.com/helmfile/helmfile/pkg/yaml"
 )
 
@@ -286,11 +287,17 @@ func ToYaml(v interface{}) (string, error) {
 }
 
 func FromYaml(str string) (Values, error) {
-	m := Values{}
+	m := map[string]interface{}{}
 
 	if err := yaml.Unmarshal([]byte(str), &m); err != nil {
 		return nil, fmt.Errorf("%s, offending yaml: %s", err, str)
 	}
+
+	m, err := maputil.CastKeysToStrings(m)
+	if err != nil {
+		return nil, fmt.Errorf("%s, offending yaml: %s", err, str)
+	}
+
 	return m, nil
 }