master: keep warning if cdef has $ds_name as a component

As kjetilho noticed on IRC :

< kjetilho> still looking at this from munin-update:  [WARNING] Service ece_cache.Global on n*...no/n*..no:4949 returned no data for label articlexml_ratio
< kjetilho> shouldn't _update_rrd_files skip services which has set .cdef in config ?
< kjetilho> sorry, skip ds_names where ds_var is "cdef"
< TheSnide> kjetilho: nope.
< kjetilho> what should the plugin do, then?
< kjetilho> to make the warning go away
< kjetilho> ah right.  so we have stuff like ntp_offset.delay.cdef delay,1000,/
< kjetilho> so I guess we could keep the warning if the cdef has $ds_name as a component