graphite: remove $::lsbdistcodename branch

Depending on $::lsbdistcodename to find the path to manage.py isn't very
pretty, nor very future proof. Fortunately, the packages are shipping
/usr/bin/graphite-manage that does all this for us, so use that instead.

Change-Id: I08ad2334554aa1ea646f31bb78f5b3ffadc721b0