[cern] Add host certificate setup, uniform cern-setup
Add configuration of the cern-hostcert atomic system container, filling in /etc/grid-security with the key and cert pem files.
As we rely on the host keytab for this, move all the CERN certificate related setup to one file (cern-setup.sh), and wait for the keytab to be properly set before proceeding to the host certificate configuration.