Keep NSS trust flags of existing certificates
Backup and restore trust flags of existing certificates during CA
installation. This prevents marking a previously trusted certificate
as untrusted, as was the case when CA-less was converted to CA-full
with external CA when using the same certificate.
https://fedorahosted.org/freeipa/ticket/5791
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>