Fix filebeat::prospector::fields_under_root (#205)
Fix filebeat::prospector::fields_under_root (#205)

`filebeat::prospector::fields_under_root` had hardcoded value instead of following global variable set for whole module `filebeat`
MiamiOH/puppet-filebeat,pcfens/puppet-filebeat,pcfens/puppet-filebeat,MiamiOH/puppet-filebeat