Sets user to nobody on EL6

As raised in the following PR the user nobody is used rather than
ganglia on EL6.

https://github.com/jhoblitt/puppet-ganglia/pull/17

This change updates the default parameter value for the gmetad user and
alters the gmond template to reflect this.
Merge pull request #19 from george-b/el6_fixes

Sets user to nobody on EL6