Adjust number of unicorn workers based on Geo processes
We saw in the Geo testbed (gitlab-com/infrastructure#2951) that we ran out of memory due to all the other processes (e.g. secondary PostgreSQL process, Geo Log Cursor, etc.) running on the secondary. We lowered the number of Unicorn workers from the automatically-detected 9 to 6. We should make Omnibus aware that we need to reserve additional memory to avoid this problem in the future.