scion-pki: generate scripts for participating parties in TRC ceremony

[lukedirtwalker]

Add the capability to generate simple bash scripts for participating parties in a TRC ceremony that are tailored to their setup.
These scripts should be simple to audit and run for participating parties.

The goal here is that participating parties do no longer have to copy-paste their commands from the documentation, but that they can invoke scion-pki to create scripts for them that they can run in the end.

This leads to less mistakes, and simpler execution of the TRC ceremony.