Patch fts-cron and renew-fts-cronjob to improve input methods

[goseind]

Motivation

While trying to create a valid x509up certificate out of a grid robot certificate from a service account at CERN to connect Rucio to FTS, I had the following issues with the ft-cron container:

• the renew script requires specific values for the cert and key file
• permission on the files might not be sufficient for voms-proxy-init
• a password input is required for voms-proxy-init

In general, the helm input and handling of different VOs and their respective certificates should be improved.

With this issue, I might also tackle #177

Modification

I suggest changes to the helm chart of the FTS cronjob and script to allow the use of different filenames, as well as a third input option for a password.
Also add additional documentation for the FTS Cron (Container).

Expected results

A improved helm chart and script, so that users do not need to worry about file names and can provide an additional secret for their PEM password.

[goseind]

Minor adjustments to the default script as well as adding the option to provide a grid passphrase. So far no solution found for cert and key filenames to be read from the secret and used as environment variables in the container - the easy solution here is to add this to the README so users are aware of the requirements.