Reimplement !unattended mode support as mainline feature
settings.yaml contains :unattended set to true by default, which enables provisioning related features during initialisation. Turning it off is meant to put Foreman into a reporting-only mode, suitable for use as a dashboard, but it's notoriously unreliable and untested.
Many major releases contain regressions because !unattended users are a minority and all developers and tests run with unattended enabled in order to test all features.
I'd suggest that we should remove this global setting and make sure Foreman can operate sensibly and be well-tested with both reporting-only and provisioning usage in a single, default configuration.
The main difference was always the New Host page, which had significant behaviour differences around OS requirements and the tabs shown. When in unattended mode, it is only possible to create a managed host in the New Host form.
Related is that there's an inconsistency in use of the host "managed" flag. Generally when unattended is enabled and hosts are !managed, Foreman acts similarly to !unattended mode. We should probably migrate hosts created under !unattended to being !managed hosts and then have Foreman work well with both managed and !managed hosts.
Related again, consider how Host::Managed should be split up or renamed when it's an unmanaged host.
Refs #10413 - Deprecate setting unattended: false
This setting is rarely used and complicates the code quite a lot. With
the introduction of host registration, there is a path for any remaining
users who had been using Foreman only for inventory to add their hosts.