This variable helps to setup all Security Products with a single
variable. It has numerous advantages over the previous version:
- The var can be set up in `.gitlab-ci.yml` or in the UI
- That means users can define it at the group level directly if needed
- It flattens the paths used (sometimes it was with /analyzers/,
  sometimes not)
- A single variable can now define all the base paths at once

This change is especially useful for air-gapped environments, where all
the images are generally duplicated locally. Having different prefixes
is creating a lot of plumbing just to get started.

Note that we need to create
registry.gitlab.com/gitlab-org/security-products/secure-bundle before
merging this.

refs gitlab-org/gitlab#209258 and gitlab-org/gitlab#209846