kompose should give warning on providing blank environment variables

[surajnarwade]

Sometimes docker-compose files have environment variables which are not set, In that case, we should give warning to user.

inspired from https://github.com/redhat-developer/opencompose

[kadel]

I don't know if this makes sense for docker-compose.
environment variables has one additional feature in docker-compse.
If value is not set for in docker-compose file, than value is taken from environment that runs docker-compose command.

[cdrage]

@kadel @surajnarwade If a value isn't provided, docker-compose will retrieve it from the host machine.

Should we close this issue?

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