Publish GlueCEPolicyMaxSlotsPerJob attribute.

Get the value for GlueCEPolicyMaxSlotsPerJob by iterating through the list of available parallel environments to get the minimum (most restrictive) value for the 'slots' parameter.

https://ggus.eu/index.php?mode=ticket_info&ticket_id=82902
https://savannah.cern.ch/bugs/index.php?95183