feat: configurable common path prefix for values files (#408)

Removes duplicated declaration path for cleaner helmfile, for example, this configuration.

```
- chart: projectABC
  name: {{ .Environment.Name }}-projectABC
  namespace: abc
  version: {{ .Environment.Values.projectABC.chart.version }}
  valuesPathPrefix: ../values/{{ .Environment.Name }}/
  values:
  - common.yaml
  - project-abc.yaml
  - platform.yaml
  - ...
  secrets:
  - secrets.yaml
```

state/state.go

@@ -109,6 +109,8 @@ type ReleaseSpec struct {
 	// The 'env' section is not really necessary any longer, as 'set' would now provide the same functionality
 	EnvValues []SetValue `yaml:"env"`
 
+	ValuesPathPrefix string `yaml:"valuesPathPrefix"`
+
 	// generatedValues are values that need cleaned up on exit
 	generatedValues []string
 }
@@ -1091,7 +1093,8 @@ func (state *HelmState) namespaceAndValuesFlags(helm helmexec.Interface, release
 	for _, value := range release.Values {
 		switch typedValue := value.(type) {
 		case string:
-			path := state.normalizePath(typedValue)
+			path := state.normalizePath(release.ValuesPathPrefix + typedValue)
+
 			if _, err := os.Stat(path); os.IsNotExist(err) {
 				return nil, err
 			}
@@ -1129,7 +1132,7 @@ func (state *HelmState) namespaceAndValuesFlags(helm helmexec.Interface, release
 		}
 	}
 	for _, value := range release.Secrets {
-		path := state.normalizePath(value)
+		path := state.normalizePath(release.ValuesPathPrefix + value)
 		if _, err := os.Stat(path); os.IsNotExist(err) {
 			return nil, err
 		}