NSSDB: use preferred convert command
    
    After further testing, Kai Engert proposed to use -N with -f -@ to
    convert a NSSDB from DBM to SQL format.
    
    https://fedoraproject.org/wiki/Changes/NSSDefaultFileFormatSql#Upgrade.2Fcompatibility_impact
    
    https://pagure.io/freeipa/issue/7049
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>