Introduce placement microversion 1.18 with a new ?required=<trait list>
query parameter accepted on the GET /resource_providers API. Results
are filtered by providers possessing *all* of the specified traits.
Empty/invalid traits result in 400 errors.
Change-Id: I8191c9a390cb02b2a38a3f1c6e29457435994981
blueprint: traits-on-list-resource-providers