Ensure custom certificates work in FIPS builds

What does this MR do?

Ensure custom certificates work in FIPS builds.

1. Bundle cacerts always, irrespective of openssl being bundled or not.
2. Use available c_rehash. If openssl is bundled, use that. Else try to find the system one and use that.
3. Set SSL_CERT_DIR for all services. This MR adds it to gitlab-rails and gitlab-exporter

Testing

1. Setup an external Redis using self-signed certificates in a VM. (For now, you might have to monkey patch Redis conf to make it work under TLS. !5770 (merged) should improve this).
2. In a FIPS machine, install the FIPS build from this MR.
3. Add the root/intermediate CA certificates of the Redis VM to /etc/gitlab/trusted-certs in the FIPS VM
4. Run reconfigure in FIPS VM
5. Confirm that the certificates added to trusted-certs are hashed and symlinked in /opt/gitlab/embedded/ssl/certs.
6. Confirm that GitLab <=> Redis interaction works fine. (Logging in should work, for example.)

Related issues

Closes #6317 (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