Replace git submodules by an Ansible requirements file

[gerryd]

We are currently using git submodules for importing external roles:

• ansible/playbooks/roles/rsyslog
• ansible/playbooks/roles/paulfantom.restic
• ansible/playbooks/roles/caddy-ansible.caddy
• ansible/playbooks/roles/fosdem.bind
• ansible/playbooks/roles/nickjj.docker

Maybe we should switch to a requirements file?

[bastischubert]

switched to requirements file. will work on the prometheus roles in another issue the next days