PWGHF: add example to modify on the fly ini in run script

[fgrosa]

@stefanopolitano this PR provides an example on how to modify the MC configurations in the .ini file to take them from any tag in CVMFS and avoid to port the configurations in all the tags, as discussed with @chiarazampolli.
To save the modified .ini file for bookkeeping, we can specify it as argument when running the grid_submit.sh (or at least I could not find a better solution).

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[stefanopolitano]

Hi @fgrosa and @zhangbiao-phy! Thanks a lot for the PR and the work. I don't have comments on this. Approved.