This series implements the referenced blueprint to allow for specifying
custom resource provider traits and inventories via yaml config files.
This fourth commit adds the config option, release notes, documentation,
functional tests, and calls to the previously implemented functions in
order to load provider config files and merge them to the provider tree.
Change-Id: I59c5758c570acccb629f7010d3104e00d79976e4
Blueprint: provider-config-file
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