This commit switch most functions in kubelet_stats.go to use the new API.
However, the functions that perform one-time resource usage retrieval remain
unchanged to be compatible with reource_usage_gatherer.go. They should be
handled separately.

Also, the new summary API does not provide the RSS memory yet, so all memory
checking tests will *always* pass. We plan to add this metrics in the API and
restore the functionality of the test.