Enable YAOOK_BACKUP_SHIFTER_WARM_KEEP_MAX for MySQL and OVSDB sts

Previously, it was not possible to set the variable YAOOK_BACKUP_SHIFTER_WARM_KEEP_MAX as a property of a MySQL stateful set. This change allows to set an integer value for backup.backupShifterOptions.warmKeepMax for all backup specs (Openstack deployment CR and OVSDB) and enables the passing of the value to the environment variable “YAOOK_BACKUP_SHIFTER_WARM_KEEP_MAX” of the backup shifter pod.