Service selector incorrect for single instance

When we set replicas: 1 in the Dragonfly Custom Resource, it turns out that the pod is lacking the `role: master` label.

As a workaround, we can explicitly set that label in the CR spec, but the service selector should not require `role: master` label being present when replicas == 1.

No file or test is named. Start by tracing the Go reconciliation path that builds the Service selector for a Dragonfly Custom Resource with replicas: 1; verify how the selector relates to pod labels. Done means the single-instance Service selects the pod without requiring role: master, while the workaround is no longer needed.

Written by the indexing model from the issue text.