Currently, the validation logic validates fields in an object and supply default
values wherever applies. This change factors out defaulting to a set of
defaulting callback functions for decoding (see #1502 for more discussion).
* This change is based on pull request 2587.
* Most defaulting has been migrated to defaults.go where the defaulting
functions are added.
* validation_test.go and converter_test.go have been adapted to not testing the
default values.
* Fixed all tests with that create invalid objects with the absence of
defaulting logic.
Create a new MetadataAccessor interface that combines both
and use it where previously latest.ResourceVersioner and SelfLinker
were being used.
Adds Namespace to the get/set interface. Adds TODO about future
fast path for metadata (as per thockin's comment)
