Backport Pulp support to 18.5

What does this MR do and why?

Describe in detail what merge request is being backported and why

Backport all required merge requests to support Pulp.

• Refactor package repository for packagecloud (!8896 - merged)
• Pulp repository implementation for uploading pa... (!8900 - merged)
• Add CI jobs to upload packages to pulp (!8884 - merged)
• Bump Omnibus builder image (!8927 - merged)
• Add RPM support for Pulp (!8928 - merged)
• Add CI jobs to upload nightly RPM packages to Pulp (!8929 - merged)
• Refactor Pulp repository to use distribution ve... (!8932 - merged)
• Add Pulp staging upload jobs for all platforms (!8935 - merged)

Related issues

Ref: gitlab-org/build/team-tasks#104

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).