[RDSv3] Configure backup_strategy.cycle

[pbol-samedi]

Description of the enhancement

We'd like to be able to configure the period/cycle for the backup_strategy of an RDS instance. The "workaround" is to disable automatic backups by setting keep_days to 0 and ignore_changes for backup_strategy and then manually changing the configuration in the OTC console, but this is bad practice. The OTC API already allows for setting a period argument.