pvl.rrd: support a separate @domain for interface target nodes, instead of using a single global domain for both the collectd host and the output rrd symlink

b3778c190aa576a9392aba5efa46d1c38ef307c1 0.6.0
edaa5d0aa57d47cc3b471903e9d1816cd252ec38 0.6.1
56ba4bef5016394977fa04df1d9508f2008f663f 0.7.0