Create a runbook for updating package repo signing keys used by Packagecloud
Related to gitlab-org/distribution/team-tasks#356 (closed) and gitlab-org/omnibus-gitlab!3946 (comment 300665820). We need a runbook describing how to create a new GPG key, how to get it into the Packagecloud instance, and how to modify Packagecloud instance to use this new key for signing package repository metadata.
Something similar to https://gitlab.com/gitlab-com/runbooks/blob/master/howto/manage-package-signing-keys.md