Add data_server entry point

[destijl]

from @scudette

I think we forgot to add an entry point for data_server.py which is a
bug, thanks for reporting. Basically we just need to add another one
like:
https://github.com/google/grr/blob/master/grr/lib/distro_entry.py#L116

pointing at the tool
https://github.com/google/grr/blob/master/grr/server/data_server/data_server.py#L843

and specify it in the setup.py:
https://github.com/google/grr/blob/master/grr/config/grr-response-server/setup.py#L18