Add interface class for backup drivers

This fix introduces an interface class to define the structure of all backup
drivers. It also renames backup/service to backup/driver.
To be backward compatible a mapping functionality is introduced to map old
backup services to backup drivers.

Implements: blueprint refactor-backup-service

Change-Id: Ic3fca567111f4bd1b221689c73cd5c3bab4a777b
Merge "Add interface class for backup drivers"