use string for excludeFromBackup label

follow-up on !22 (merged) this MR makes sure the excludeFromBackup is a string. so volumes should be exluded when excludeFromBackup is set to "true" not to true

ping @jan-eike.schnabel

assigned to @philipp.sommer

enabled an automatic merge when the pipeline for 65e07022 succeeds

merged

mentioned in commit abf3156d

changed the description

mentioned in commit hcdc/kubernetes/charts/thredds@0a205691

mentioned in commit hcdc/kubernetes/charts/nginx@4c05e673