openPDS-RegistryServer expects a ping url

The [is_pds_up](https://github.com/HumanDynamics/openPDS-RegistryServer/blob/master/registryServer/apps/base/views.py#L58) function in the RegistryServer expects a `/discovery/ping` url in openPDS.

This PR adds this to the root urlconf, and it also cleans up the `openpds.views` module a bit (removing some unnecessary imports).