ipa: check if provided config file exists
    
    Add a parser check to verify config file supplied to the ipa
    command exists. Previously, invalid file paths would not results
    in any error and would just silently proceed with default config.
    
    https://fedorahosted.org/freeipa/ticket/6114
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>