slapos_cloud: Include API to renew the certificates
It is just a shortcut to call revoke and generate in sequence, allowing it to recreate the values.
TODO:
- Add (for now) one alarm to renew software instances certificates periodically (every few months)
In future (not on this merge request), we want to get slapgrid auto renewing the certificates by themselves and removing the storage of the certificates on the Software Instance Document.