Fix Constructed Inventory colliding with defaults on controller_inventory_sources

[Klaas-]

What does this PR do?

Follow up to #720 references #601

this changes the behaviour to allow modifications in constructed inventories, for example if you want to set a limit or modify source_vars.

I've updated the test constructed inventory to use both features.

How should this be tested?

Constructed inventory in tests/configs/inventory_sources.yml should work

Is there a relevant Issue open for this?

references #601

Other Relevant info, PRs, etc

#720