graphite::web: bump uWSGI workers from 4 to 8

4 is very low. We have enough things calling the Graphite render API
concurrently (users, alerts, gdash, etc.) that we often get 502s.

We could increase the value from 4 to 8 in the role class, but I think 8 works
better as a default anyhow.

Change-Id: I581bd2a9005aaeb8a54bc458c5c0c7523cfd63e2
Merge "graphite::web: bump uWSGI workers from 4 to 8" into production