change the nest level of envvals overrides for nested state files(sub-helmfiles) (#635)

We added envvals overrides in the state file via #622 two days ago:

```
helmfiles:
- name: sub.helmfile.yaml
  environment:
    values:
    - mykey: myvalue
```

This change removes the `environment` level in the above cofig, so that it looks like:

```
helmfiles:
- name: sub.helmfile.yaml
  values:
  - mykey: myvalue
``

This is an inevitable breaking change towards #361. But I wanted to break it earlier so that less folks are affected.`

Ref https://github.com/roboll/helmfile/issues/361#issuecomment-497530819

state/state.go

@@ -1477,7 +1477,7 @@ func (hf *SubHelmfileSpec) UnmarshalYAML(unmarshal func(interface{}) error) erro
 			Selectors          []string `yaml:"selectors"`
 			SelectorsInherited bool     `yaml:"selectorsInherited"`
 
-			Environment SubhelmfileEnvironmentSpec `yaml:"environment"`
+			Environment SubhelmfileEnvironmentSpec `yaml:",inline"`
 		}
 		if err := unmarshal(&subHelmfileSpecTmp); err != nil {
 			return err