Fix discovery in rolling upgrade tests

[DaveCTurner]

#33675 has improved how discovery is configured in most tests, but the rolling upgrade tests still have vestiges of the old way of doing things, particularly:

• explicitly setting minimum_master_nodes to 3 on freshly-started nodes
• manually configuring the unicast hosts list rather than relying on file-based discovery

This pertains to at least:

• :qa:rolling-upgrade
• :x-pack:qa:rolling-upgrade-basic
• :x-pack:qa:rolling-upgrade

[elasticmachine]

Pinging @elastic/es-distributed