PluginCache.get_plugin_config assumes that no more configuration is to
be processed, and therefore config is final. As such, it is validating
that mandatory parameters are set. This assumption is invalid for
workload_parameters, however, as those need to be resolved on per-spec
basis, and cannot be globally cached.
This commit adds a prameter for get_plugin_config that indicates whether
or not it should consider the config to be final.