Fix self.opts.partition vs. self.force_reinstall semantics

This is a follow-up on #8859 (closed) and #8860 (closed). Details will follow in the comments.

Feature Branch: bugfix/15590-14849-14720-14810-installer-fixes-for-3.9, installer:bugfix/15590-14849-14720-14810-installer-fixes-for-3.9

Original created by @anonym on 14720 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information