Automatic merge from submit-queue

Split conversion and defaulting

Separate conversion and defaulting.   Defaulting occurs mixed with conversion today - change the server so that the `VersioningCodec` performs defaulting on the external type during decoding.  

* Add a new method to `Scheme` - `func (*runtime.Scheme) Default(runtime.Object)` - that takes an object and performs defaulting.  
* Call `Default` during decoding and at static initialization time
* Use the new `defaulter-gen` to generate top level object defaulters (`v1.Pod`) at build time for any type that needs to perform defaulting.  
* Add tests and alter the existing code to adapt as necessary
* Fix a few bugs in conversions that depended on defaulting behavior

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Step 1 of decoupling conversion and defaulting. The generator will assist in creating top level defaulters that in a single method invoke all nested defaulters, preventing the need to recurse via reflection or conversion. These top level defaulters will be registered in the scheme and invoked instead of the nested recursion path. This will set the stage for a future generator, capable of creating defaulters from embedded struct tags on external types. However, we must gradually switch these over.

The immediate goal here is to split defaulting and conversion so that the unsafe convertor can be used to maximum potential (we would be able to use direct memory conversion for any identical nested struct, even those that must be defaulted).

The generator uses `k8s:defaulter-gen=TypeMeta` on most public packages to flag any top level type that has defaulters to get a `SetObjectDefaults_NAME` function created (types that don't have defaulters won't have functions).  This also creates a `RegisterDefaults` method that applies a default to an interface{} and returns true if the object was handled.  Existing defaults are left as is.

Add a test to verify old and new path generate the same outcomes.  Defaulter will move to gengo before this is merged, and subsequent PRs will remove defaulting during conversion and have the VersioningCodec apply defaults.