ipaserver,ipareplica: Fix Random Serial Numbers always enabled

The option _random_serial_numbers was using with the wrong type in
ipaserver_setup_ca.py and ipareplica_setup_ca.py. Therefore RSN was
always enabled.

Fixes:
- https://bugzilla.redhat.com/show_bug.cgi?id=2110523
- https://bugzilla.redhat.com/show_bug.cgi?id=2110526