enable/disable configurations: DB01_ENABLED or similar #444

Thanks for your great tool!

Description of the feature
In order to be able to define a reusable set of services to be backed up, I would like to be able to enable/disable services easily.

Benefits of feature
Configuration can stay the same (using variables). Simple boolean variable can define if service is backed up.

Additional context
This could be achieved introducing an DB01_ENABLED flag (default: true)