Add config settings to make locating openssl easier

What does this Merge Request do and why?

This MR adds a few new settings, designed to make it easier to determine the best location to the openssl binary, across different platforms.

This MR will be built upon in !2449.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.