[PM-398] Improved start.sh script

Context

We want to get rid of manual rewriting of secrets files locations after deploys, which requires more templating of start.sh

What was done

• Allow specifying secrets files which should be sourced
• Make Kerberos authentication configurable
• Use set -e in the script to fail launch when any of the commands cannot be executed, thus correctly failing the systemd unit

Review objectives

• assessment of code functionality
• assessment of code quality
• knowledge sharing