Skip CS.cfg update if cert nickname not known
After CA certificate renewal, the ``renew_ca_cert`` helper updates
certificate data in CS.cfg. An unrecognised nickname will raise
``KeyError``. To allow the helper to be used for arbitrary
certificates (e.g. lightweight CAs), do not fail if the nickname is
unrecognised - just skip the update.
Part of: https://fedorahosted.org/freeipa/ticket/4559
Reviewed-By: Jan Cholasta <jcholast@redhat.com>