Move dask config into {sys.prefix}/etc/dask

[delgadom]

See #130

We should be modifying the default dask config file so the user can set their own preferences, rather than overwriting the user's preferences.

The dask config file includes way more preferences than are in this file, so if we do overwrite this file, we should be extremely careful not to drop any default dask config, and figure out how to keep this up to date.

[brews]

I don't know your setup process too well, Is this something to handle in a start up script or would you burn this into the docker image?