Implement uid/gid in header.erb as variables to be used in rsync::server

In some cases, we need to set another UID or GID than 'nobody'. E.g. the official OpenStack Puppet-Modules need it for swift deployment. Currently it cannot implement it directly:

https://github.com/stackforge/puppet-swift/blob/master/manifests/storage.pp
https://github.com/stackforge/puppet-swift/blob/master/templates/rsyncd.conf.erb