[cern] Fix CERN Grid Certification download path

Due to a new certificate, when magnum installs the CERN certificates (even when cern_enabled is false) fails because the replcaed certificate was removed and the file cannot be downloaded.

When we trying to update-ca-trust this operation fails and cluster fails to create. Using the new file path should be a quick fix for this

Change-Id: Ie3d1e0049d859828377963ef25d8e9dcb79a7dd5