Make metadata workers configurable

In Icehouse the default changed from 1 to number of cores.
This default doesn't make sense (no reason to run 64 workers
on a 64 core compute node for instance)

Note the default of 0 actually means 1
(that's why I add 1 to the value for the nagios check)

Change-Id: I1fd009821d174095361e0fee5f1bc32d49d9575b