Upgrade to td-agent v4 on older Puppet installs

The fluentd_gem provider for installing fluentd plugins uses
Puppet's built-in gem provider. Unfortunately, that gem provider
on our Puppet version is not compatible with td-agent's new embedded
Ruby since it includes the no-rdoc and no-ri flags deprecated and
replaced by the no-document flag. This install function override
replaces the flags and might be removable after a Puppet upgrade.

manifests/params.pp

@@ -19,7 +19,7 @@ class fluentd::params {
   $user_group  = 'td-agent'
   $user_groups = ['adm']
   # repo params
-  $repo_version = 3
+  $repo_version = 4
 
   case $::osfamily {
     'Debian': {