Use extensions template from NSS
Drop certmonger's custom extension template and use the sequence of X509v3
extensions template from NSS.
The certmonger template had a bug that caused certmonger to create CSRs
with invalid DER. It was encoding extension's critical element even for
default value FALSE.
Fixes: https://pagure.io/certmonger/issue/223
Signed-off-by: Christian Heimes <cheimes@redhat.com>