Use relative path for database config files

Currently the catalogue and scheduler files are hardcoded to the following absolute paths:

  local catalogue_credentials="/home/cirunner/shared/CTA/continuousintegration/orchestration/pgsql-pod-creds.yaml.example"
  local scheduler_credentials="/home/cirunner/shared/CTA/continuousintegration/orchestration/sched-vfs-creds.yaml.example"

This has some portability issues so we should change this to a relative path instead.