helmfile as of today ensures that all the targeted helmfile.yaml to have the specified environment defined in it.
That is, `helmfile --environment prod -f helmfile.d/ sync` fails if any helmfile under `helmfile.d/` is missing the `production` environment.
This changes the validation logic, so that helmfile fails only when all the helmfiles miss the environment.
Resolves#279
KUOKA Yusuke