Agent: Directories and permissions

[stefanigel]

The topic of directories and permissions needs a holistic view and decision. Therefore all relevant issues are grouped in this epic.

The holistic view & decision is handled by an ADR

• Agent should check on startup if important directories are readable/writable #45
• Create missing directories required by the agent #105
• Create missing directories required by the services/operators #271
• Ability to set file permissions on config files #20
• Adapt ownership of directories #269
• Create the service user if non-existent #106