previously gather_subset=['!all'] would still gather the
min set of facts, and there was no way to collect no facts.
The 'min' specifier in gather_subset is equilivent to
exclude the minimal_gather_subset facts as well.
gather_subset=['!all', '!min'] will collect no facts
This also lets explicitly added gather_subsets override excludes.
gather_subset=['pkg_mgr', '!all', '!min'] will collect only the pkg_mgr
fact.
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