Add support for a HelperScript option

[pllopis]

This MR adds support for a HelperScript option in the [api] config section to be run at the first "auks cred get" and after each renewal.

This has been tested to work in a test environment just fine. Would like to send this out to get feedback on the approach/implementation. This is a feature that we're going to be running in a production environment with AFS and EOS soon, so I can give more info on how well this works in the near future, if there are such concerns. Long term we would like not have to patch auks to achieve this, of course :)

I think I've added the relevant bits of docs, headers and comments, but let me know if I've left anything out that should be in there as well.

[hautreux]

Thanks for you work on auks. Great addition.

This repo is no longer maintained, please go to https://github.com/cea-hpc/auks to find the official version and use that for your PR instead.