[BZ 1115415] Make resources in the discovery q possible to manually add

During resource add we ignored resources already existing in the inventory.
Resources in the discovery queue ARE present in the inventory with the
status set to NEW.

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This behavior was present in the codebase since the dawn of time so I am
a little bit nervous about changing it. Not sure about the (c|m|i)time values
as well as whether to create history entry for such resources, etc.

Also I still need to investigate how this behaves for the ephemeral
uninventoried resources before they asyncly disappear from the database.

What are your thoughts?