hydra-module: Use PermissionsStartOnly in init.

Oops, forgot to add this in f75509099a558ecfdd2f60a036c6e71de430920a.

This is necessary because we actually want to run the `preStart` script as `root` (because it chmod/chowns stuff and also needs to create the database using PostgreSQL's superuser) and the actual creation of the database as user `hydra`.