Fix "Variable access via 'log_dir' is deprecated" warnings

This fixes the following message, which results when declaring `munin::node` in Puppet 3.2.4:

    Warning: Variable access via 'log_dir' is deprecated. Use '@log_dir' instead. template[/tmp/vagrant-puppet/modules-0/munin/templates/munin-node.conf.erb]:5
    (at /tmp/vagrant-puppet/modules-0/munin/templates/munin-node.conf.erb:5:in `result')