Allow more granular Couchbase Server upgrades

Couchbase Server upgrades are triggered by updating the image name field (couchbaseclusters.spec.image). The single value applies to the entire cluster, and there is no way to only upgrade a subset of the nodes.

If we allow the image name to be (optionally) specified in multiple places in the Cluster definition (e.g. Server Groups, Server Classes), or with some other kind of node selector (potentially via an annotation), this would allow fine-grained control of which nodes would be upgraded, and crucially, would allow the Operator to be in a reconciled state after each round of upgrades.