Ironic nodes that are not available or operable have 0 values for vcpus,
memory_mb, and local_gb in the returned dict from the Ironic virt driver's
get_available_resource() call. Don't try to save these 0 values in the
placement API inventory records, since the placement REST API will return an
error. Instead, attempt to delete any inventory records for that Ironic node
resource provider by PUT'ing an empty set of inventory records to the placement
API.
Closes-bug: #1651678
Change-Id: I10b22606f704abcb970939fb2cd77f026d4d6322