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.
    
    https://fedorahosted.org/freeipa/ticket/4899
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>