Use graphite location variable for rrd directory

Looks like a merge conflict wiped out using the graphite location variable for the rrd directory.
Originally implemented at:
https://github.com/hectcastro/chef-graphite/commit/6065999c1dcec51a6cd28a2724e7d9d135c2f67c#diff-1d369fdbbba07bdf9dd536edf76f0954

Looks like it was accidentally wiped out in a merge conflict here:
https://github.com/hectcastro/chef-graphite/commit/4e9603ad2a3c59a6af6d663a2acd9784e6cf01c1#diff-1d369fdbbba07bdf9dd536edf76f0954