Fix cert file creation during CA-less installation
    
    When writing extracted certs and keys to the file, we opened the
    same file at a different spot but the original file position
    indicator would not be moved when the certificate is written there.
    The result is that the certificate gets rewritten by the private
    key.  This commit fixes it.
    
    Fixes: https://pagure.io/freeipa/issue/7118
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>