add prospectors_merged argument
add prospectors_merged argument

When set to "true", this allows one to set different prospectors on
different hiera levels, and have them merged. The implementation
still allows setting prospectors without hiera. Setting the argument to
true and using manifest instantiation also works, since the hiera_hash()
default uses the parameter itself as fallback.

pcfens/puppet-filebeat,MiamiOH/puppet-filebeat,digitalmediacenter/puppet-filebeat,MiamiOH/puppet-filebeat,pcfens/puppet-filebeat,digitalmediacenter/puppet-filebeat