Allow incomplete bucket for Secure Files in object storage configuration

What does this MR do and why?

This MR applies the suggestion in #414673 (comment 1546146600) to add the ci_secure_files config to SUPPORTED_TYPES and ALLOWED_INCOMPLETE_TYPES so that the consolidated storage config can be used, but not produce an error if the bucket configuration is missing.

This change resolves the issues that caused the original change to be reverted Revert adding ci_secure_files to consolidated o... (!122905 - merged) and allows us to add support for consolidated object storage without introducing a breaking change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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