Publish max slots per job for GLUE/GLUE2.

Get the value for GlueCEPolicyMaxSlotsPerJob and
GLUE2ComputingShareMaxSlotsPerJob attributes by
iterating through the list of available parallel
environments to get the minimum (most restrictive)
value for 'slots' parameter.

https://ggus.eu/index.php?mode=ticket_info&ticket_id=82902
https://savannah.cern.ch/bugs/index.php?95183