Considering adding support for env variables

Hi, I was playing with distributed and for both dask-worker and dask-scheduler you have to specify an IP:PORT (generally).

Considering the use with docker images and containers I think it would be useful to use also env variables.

Would you consider to add something like:
```
DASK_WORKER_IP=your_ip
DASK_WORKER_PORT=your_port
```

```
DASK_SCHEDULER_IP=your_ip
DASK_SCHEDULER_PORT=your_port
```
to specify them?