Extract operator defaults to a separate func

[ChunyiLyu]

Note to reviewers: remember to look at the commits in this PR and consider if they can be squashed

Summary Of Changes

• extract operator defaults to a separate func to keep Reconciler shorter and more concise
• extract operator defaults controller tests to dedicated test unit as well
• use updateRabbitmqCluster() since the update logic is shared for updating these operator defaults

Additional Context

Local Testing

Tested in integration controller tests suite