When helmfile is run with `--environment NAME` and there was a base hemlfile that misses `environments`, helmfile had been trying to load env values for NAME and failing.
A base helmfile is allowed to reference values from within itself, but that's optional. In other words, a base helmfile that misses the env is okay as long as it doesn't self-reference env values.
So, this change allows missing env and env values while loading base helmfile. After loading, a base helmfile can fail due to referencing missing env values, but that's okay.
Fixes#1008
07c42474cc