csrgen: Add code to generate scripts that generate CSRs
Adds a library that uses jinja2 to format a script that, when run, will
build a CSR. Also adds a CLI command, 'cert-get-requestdata', that uses
this library and builds the script for a given principal. The rules are
read from json files in /usr/share/ipa/csr, but the rule provider is a
separate class so that it can be replaced easily.
Reviewed-By: Jan Cholasta <firstname.lastname@example.org>