Ability to set "privileged" flag (boolean) from environment var

```
version: '2'
services:
  mongo:
    image: mongo
    privileged: $PRIVILEGED
```

But I got this error output:

```
ERROR: The Compose file './docker-compose.yml' is invalid because:
services.mongo.privileged contains an invalid type, it should be a boolean
```

Might be accept the value `"true"` will make sense..

Thanks
