Skip systemd service management in check mode

Default behaviour has been changed in ansible 2.10, running this role in check
mode was failing with the following error:

fatal: [XXXXX]: FAILED! => {"changed": false, "msg": "Could not find the requested service node_exporter: host"}

See https://github.com/ansible/ansible/issues/72721 and
https://github.com/ansible/ansible/pull/68136 for more information.