Operator Backup and Restore scope definition

This issue specifies the scope of the backup and restore functionality of 3scale through 3scale-operator.

From the engineering team we have defined the following scope for the functionality:

• A backup performed from a 3scale release X.Y installation can only be restored in a 3scale release X.Y installation
• This implies that a user cannot perform a backup from a 3scale 2.7 installation and restore it in a 3scale 2.8 installation
• Both the backup and restore of a 3scale installation require the installation to be performed with the 3scale operator.
• This implies that the functionality is NOT designed/suited for migrating from a template-based installation to an operator-based installation or viceversa
• backup/restore is only compatible when both the origin and destination installation are configured with external databases (at the moment of writing this the external databases are backend-redis, system-redis and system database)
• This implies that backups and restores where some or both installations were in "embedded" databases cannot be performed
• The backup and restore of the external databases is out of the scope of the functionality and it is responsibility of the user

We will add more text/modify it if there are changes on the scope or and/or more detail is added.