Fix certificate retrieval in ipa-replica-prepare for DL0
    
    The NSSDatabase object doesn't know the format of an NSS database
    until the database is created so an explcit call to nssdb.create_db.
    
    https://pagure.io/freeipa/issue/7469
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>