switch carbon-cache to systemd

the init.d script didn't seem to work on my debian testing machine which has
systemd as its init system. since systemd is the default on debian now anyways
this shouldn't be a problem.

the error message I got when restarting carbon-cache manually was this:

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root@dev1:~# /etc/init.d/carbon-cache restart
[....] Restarting carbon-cache (via systemctl): carbon-cache.serviceJob for carbon-cache.service failed. See 'systemctl status carbon-cache.service' and 'journalctl -xn' for details.
 failed!
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so somewhere system hooked into the init.d script and I suspect the sudo-to-
graphite-and-calling-the-script-again was what broke. systemctl status and
journalctl as well as the system log all did not reveal details on the exact
problem - and so I migrated carbon-cache to systemd

there's one issue remaining: carbon-cache must be called with --debug or else
the process will exit with 0 and no carbon-cache will be running, no ports and
will be open.