iso-manager script should fail fast with clear error message when bad archive entries are detected in bootstrap.yaml

[thomasdanan]

Component:

'iso-manager'

Why this is needed:

When upgrading to a new metalk8s version, we first need to import the new metalk8s.iso.
When doing so, the iso-manager script may fail with the following error:

Rendering SLS 'metalk8s-2.7.0-dev:metalk8s.salt.master.installed' failed: Problem running salt function in Jinja template: Failed to run isoinfo: isoinfo: No such file or directory. Cannot open 'metalk8s.iso'. Cannot open SCSI driver. isoinfo: No such file or directory. Unable to open metalk8s.iso; line 34

Actually, this is because of the bootstrap.yaml file was having a bad entry (a relative path to an iso) in the archives section.
Don't really know how this entry was added to the config file but since it can be edited manually, it may happen that bad archive entries are present in the file.

What should be done:

In such a situation, the iso-manager script should abort immediately and provide the administrator with a clear error message for him to understand that the bootstrap.yaml contains some wrong archive entries and that it should be removed.
Ideally, the iso-manager script starts with listing the archive entries and make sure they are all valid (i.e. they point to existing iso file on disk). If one of them is not valid it failed and displays the error message.

Implementation proposal (strongly recommended):

Test plan: