Fixes #36741 - Check Ubuntu version more explicitly

[bastian-src]

If no minor version is defined, the function minor.to_i makes it a "0". Accordingly, Ubuntu 20.04 is interpreted as "no subiquity/Autoinstall" - but, it should not assume a specific version here.

Therefore, is_subiquity should return ´true` in this case (especially, since all 20.04.x versions support subiquity/Autoinstall provisioning, but only specific ones allow Preseed/debian-installer still).

[theforeman-bot]

Issues: #36741

[bastian-src]

@sbernhard I've just rebased to latest develop. Tests seem fine.

[ekohl]

When is there ever not a minor?

[goarsna]

Currently the official foreman documentation states, that if you create an Ubuntu operating system you should assign a major version (e.g. 20.04) and a minor version (e.g. 6). And I think this also is the case for the documentation of all other operating systems. So in most cases there should be a minor version set.

But there are cases where one might want to create an Ubuntu operating system without minor version. For example this is currently required for Host Registration of Ubuntu hosts, because the Fact parsers only provide information about the major version of Ubuntu to foreman (see my issue and the corresponding PR for reference). So for Host Registration to work, I have to create an Ubuntu operating system with major version 20.04 and empty minor version.
If I then provision a host using this Utbuntu 20.04 operating system, I won't be able to install the latest Ubuntu 20.04 wich is Ubuntu 20.04.6 as subiquity/autoinstall provisioning has to be used for it, but it won't be detected to be used.

This would be fixed by this change introduced by @bastian-src.

[sbernhard]

LGTM. Thank you.