Resolve env without value to client's value

For parity with the Docker CLI, allow environment variables to be set
without values and be automatically resolved to the value on the client.
This is a direct port of opts.ValidateEnv from https://github.com/dotcloud/docker/blob/170609e36957fc044723789bc66918bd7d189702/opts/opts.go#L123-L129 and is applied during container config.