Is it necessary to configure "round-robin" DNS in the Primary DNS or can a secondary DNS be utilized?

This can be done in any DNS server, primary or secondary. Swarm doesn't need DNS; it is for the benefit of the Swarm clients. The round-robin record shown below would allow for clients one method of distributing their initial requests across the cluster:

swarm  0 IN A 192.168.1.101

       0 IN A 192.168.1.102

       0 IN A 192.168.1.103

       0 IN A 192.168.1.104