Include /etc/munin/munin-conf.d for additional configuration

When munin.conf is created by chef, its nice to have the munin-conf.d included so one can add custom nodes with manual config. Like for snmp-nodes or legacy-machines that aren't findable on chef.

This PR depends on current master, not on one of the other PRs from me:)

Ticket COOK-4133 in tickets.opscode.com