Mark the 'installed' state of a node with a special flag

Nodes now have an 'installed' flag that gets set by installers doing a GET against stage_done_url('finished'). An 'installed' node is protected from reinstallation by default as it will now always boot into the noop installer, even if no policy is assigned to it.

it would also be possible to trigger the not_installed -> installed transition through metadata in the installer: add a field 'installed_after' or similar to the installer YAML files and check here if the boot_count has advanced past that. This might be a friendlier mechanism for some installers, though for others, the current mechanism is more accurate (e.g. the various Linux installers)