Configure default value for backup objects to keep in metal-roles

[simcod]

A default value for backup objects is configured here:

backup-restore-sidecar/pkg/constants/constants.go

Line 5 in c859648

DefaultObjectsToKeep = 20

This issue suggests to configure the default value via ansible in metal-roles. The number of objects saved in the bucket could be increased or discussed at this point.
A proposal can be found in the following PR.