certdb: never use the -r option of certutil
The -r option makes certutil output certificates in DER. If there are
multiple certificates sharing the same nickname, certutil will output
them concatenated into a single blob. The blob is not a valid DER
anymore and causes failures further in the code.
Use the -a option instead to output the certificates in PEM and convert
them to DER on demand.
https://fedorahosted.org/freeipa/ticket/5117
https://fedorahosted.org/freeipa/ticket/5720
Reviewed-By: David Kupka <dkupka@redhat.com>