GlideinWMS

Continuation of the work in [#21885]

The running of periodic scripts in singularity:
- HTCondor may provide a mechanism to run startd_cron scripts in singularity
- the current mechanism (script re-invoking itself) may not work in startd_cron

It could actually work with little or no changes but should be verified, several parts could break, especially the function adding the script in the condor configuration and the ability to access the environment and the files (importing singularity_lib.sh, ...)