Broaden Service Selector

[eberlep]

Since the spilo-role=standby_leader label is not consistently applied to standby instances, the selector in our Service does not always work.

As the Service ressource does not support set-based selectors (yet?), we cannot use the spilo-role in (master, standby_leader) selector that we already use in other places.

Since we cannot change the selector to make sure it only matches one pod, we broaden the selector (by removing the spilo-role part) and make sure the numberOfInstances is 1 for standby instances. This, however, should already be covered in the backend.

[eberlep]

Another idea: For standby databases, select the first pod directly and don't use and spilo-role label. In a perfect world, standby databases should consist of a single instance anyway (which, by definition, will always be called ...-0). If there happen to be more instances, we would not round robin between those (as would be the case when simply selecting application=spilo pods), and if one would downscale the standby, existing connections to it would be kept open.