Workers created by `LocalCluster` have dashboards disabled by default

When starting a cluster with `LocalCluster` it appears that the workers have their dashboards disabled by default as the default `worker_dashboard_address` is `None`.

https://github.com/dask/distributed/blob/f2b09c6b55188f6b633cba5422454b689b0fd7b4/distributed/deploy/local.py#L177

This is counter to starting a worker from the command line where the dashboard [is enabled by default](https://docs.dask.org/en/latest/setup/cli.html#cmdoption-dask-worker-dashboard).

Setting the `worker_dashboard_address` to `0` in `LocalCluster` results in the dashboard running on a random high port. This is the same behaviour as the command line.

```python
cluster = LocalCluster(worker_dashboard_address=0)
```

Therefore I propose we change the default value for `worker_dashboard_address` in `LocalCluster` to `0`. User's can still disable the dashboard by explicitly setting `worker_dashboard_address` to `None`.