Introduce a global_handlers role to collect handlers needed by different roles

[ThiloSolbrig]

• Certain handlers might be useful for more than one role (e.g., restarting the server after package updates or kernel parameter changes). Instead of defining them redundantly in each role, a new role is introduced to collect those roles. Any role which needs those handlers, e.g. can import them via a role dependency.
• As a first shot
  • "restart server" handler will be moved there.
  • "restart if needed" handler will be introduced, which checks the system for reboot recommendation, e.g. after package updates
  • "restart server" handler will be changed from forced reboot to configurable reboot (actually reboot or just display reboot advisory only). This is because it might be inacceptable to accidently reboot a production system, for instance.

[ThiloSolbrig]

Implemented by: #455