roles/ipaserver: Allow deployments with random serial numbers

Since FreeIPA version 4.10 it is possible to deploy servers that use
Random Serial Number v3 support for certificates.

This patch exposes the 'random_serial_numbers' parameter, as
'ipaserver_random_serial_numbers', allowing a user to have random serial
numbers enabled for the domain.

The use of random serial numbers is allowed on new installations only.