This will allow more specialized usage of upgrade run command, similar
to patterns used with 'external-upgrade run', e.g. during
reprovisioning of nodes.
Also, validation on '--skip-tags' is removed because we should no
longer assume what tags are defined by the upgrade tasks, and we
should allow more free-form usage.
Change-Id: Icf672cc0db50ea6daa57d3c5abe131c82553fc5a
Implements: blueprint upgrades-with-os