Add deamon variable to cache key

Add deamon variable to cache key

This allows for two identically named rrd's on two different rrdcache daemons
to be graphed on the samen graph. Without this patch the following

rrdtool graph - --start=1478559600 --end=1478685600 --width=1000 --step=300 --imgformat=JSONTIME \
  DEF:rrd1=aname.rrd:ds_name:AVERAGE:daemon=rrdcache1 \
  DEF:rrd2=aname.rrd:ds_name:AVERAGE:daemon=rrdcache2

would not actually fetch any data from rrdcache2, but rrd2 would reuse the data from rrd1,
since the current logic determines them to be equal.

With this patch rrd2 data is actually fetched from rrdcache2.