Invoke DELETE on pod prepare-downscale path if any POSTs failed

[seizethedave]

This addresses a bug in rollout-operator where:

1. Kubernetes receives a request to downscale a statefulset by X hosts.
2. The prepare-downscale admission webhook attempts to prepare X pods for shutdown by sending an HTTP POST to their handler identified by the grafana.com/prepare-downscale-http-path and -port annotations.
3. At least one of these requests fails. The admission webhook returns an error to Kubernetes, so the downscale is not approved.
4. 💥 But some hosts may have been prepared for downscale. 💥

This PR adds cleanup logic to issue DELETE requests on all involved pods if any of the POSTs failed. Notes:

• DELETE calls are attempted once.
• DELETE failures are logged but otherwise ignored.
• For simplicity, we'll invoke DELETE on all of the pods involved in the scaledown operation, not just ones that received a POST.

This doesn't fix the similar issue where replica count changing from 10->9->10 leaves that one pod prepared for shutdown. (But that's in the works.)

[pracucci]

Let's wait for @pstibrany review here. He's the expert in this area.