graphite-web: set DEFAULT_CACHE_DURATION to 120s

Make graphite-web cache images and data for two minutes. The default behavior
is to cache for one minute. This matches the old setup on professor, which
served us well.

This change should be followed-up with a patch configuring mod_expires to set
Cache-Control/Last-Modified/Expires.

Change-Id: I863928efeb0475273e1dbff6c34ad779ee9e2933