Improve initial leader selection

By setting the scale policy to 'serial', we can improve how the initial leader is selected. It the pod is alone, it must be the leader and can label itself as such, per https://discourse.charmhub.io/t/updated-podspec-yaml-new-features/2124/16 . We can probably avoid passing the list of pending-units via pod spec entirely, and avoid the issue of needing to map unit names to pod names which would also fix lp:1903625