Exclusiveexecution - requires number of cores set

When exclusiveexecution is set in xrls the emies-es specifications requires that number of slots is set also http://www.nordugrid.org/documents/EMI-ES-Specification_v1.16.pdf

Jobs where no "count" parameter is set if exclusiveexecution = "yes" is set, will fail and give the client error INFO: Failed to submit all jobs: 500 emies:adl parsing error

On the server side though one finds in ws-interface.log: [Arc.JobDescriptionParserPlugin] [DEBUG] [1571007/16] [ADLParser] Missing or wrong value in NumberOfSlots

One suggestion was to set a default value to number of slots to 1 if none is set in the users job description.

However, I think a better solution would be do propagate the error from the server, namely "Missing or wrong value in NumberOfSlots"

Then the user would understand that the is some requirement not fulfilled in his job description.

This PR only includes the default value fix.