UPSTREAM: meta_test should not depend on runtime.TypeMeta

cherry-pick(github.com/GoogleCloudPlatform/kubernetes): 508724b1f8431b860f3c35607127a41e70f2f0ed
UPSTREAM: Add Labels and Annotations to MetadataAccessor

cherry-pick(github.com/GoogleCloudPlatform/kubernetes): 28f5d51a5f6975a575d0cd5157c114878deba2ed
Rename config.mergeMaps() as util.MergeInto()
Refactor Config obj.Labels to use meta.Accessor
Fix #286: Fix Config with nil Labels

- Fix nil Label case
- Remove util.MergeMap() dst auto allocation, forbid passing pointers
- Improve util.MergeMap() tests and its readability
- Re-enable AddConfigLabels test for Unknown Generic Object having Labels field