Add nightly jobs to use system ssl

What does this MR do?

Add CentOS 8, Ubuntu 18.04, and Ubuntu 20.04 jobs using system ssl to the nightly pipeline.

Design decisions:

1. FIPS builds are built as part of EE nightlies only, and we will ensure they are distinguishable from regular EE builds.
2. They go to fips-nightly-builds repo in package cloud.
3. The package name will be gitlab-fips, at least for now.
4. The versioning will be <latest_stable_tag>+rnightly.fips.<pipeline_id>.<commit_sha>-0. For example, the package file name will look like gitlab-fips_14.7.0+rnightly.fips.224980.733542cb-0_amd64.deb. If/when we eventually move the package name to be gitlab-ee instead of gitlab-fips, the fips substring in the version will ensure EE => FIPS jump will be considered an upgrade by the package managers.

Related issues

Closes #6576 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened