Add 'graphite-index' script and cron job

The graphite-web package ships with a shell script, 'graphite-build-search-index',
which rebuilds the index of Whisper files used by graphite-web. The Debian
script presumes that graphite-web and carbon are both running with uid / gid
'_graphite', but we prefer to keep them distinct, because graphite-web should
not be modifying Whisper files. So ship an alternative script that better
matches our setup.

Change-Id: I83a2c6dbecba7545bb6e849f4e0089d69f202f44