Fall back to c_rehash if there are multiple TLS certificates

What does this MR do and why?

This backports !8711 (merged) to 18-3-stable.

The switch to openssl rehash from c_rehash in !8306 (merged) had an unintended breaking change: certificates in /etc/gitlab/trusted-certs are no longer processed if they contain multiple certificates.

To avoid this breaking change, detect the warning and fall back to c_rehash:

!8306 (merged) excluded c_rehash, but in case openssl rehash fails we need to keep this for now.

rehash: warning: skipping godaddy.crt, it does not contain exactly one certificate or CRL

Related #9295 (closed)

Related #9304

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• 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).

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