preload pegasus app in unicorn config - #11930

Followup to #11598 and #11867.

Set preload_app=true for the Pegasus Unicorn configuration, which will cause the pegasus upgrade service to wait until it's fully booted, and return non-zero exit code if it can't start properly. This fixes the 'spawn loop' issue.

Documentation:

Using #preload_app=false (the default) means you must check if your application is responding properly after a deployment. Improperly deployed applications can go into a spawn loop if the application fails to load. While your children are in a spawn loop, it is is possible to fix an application by properly deploying all required code and dependencies. Using #preload_app=true means any application load error will cause the master process to exit with an error.

(Dashboard already uses preload_app=true)