Make needed changes in conversion package to support pluggability
pkg/runtime now has a well defined api pluggability model.
Fixes:

* Fix kubecfg test, which was writing an internal object straight to yaml
* Address review comments
add PluginBase to fuzz functions
Remove custom scheme stuff

As separate commit to make it easy to get back-- I'm not completely
convince we won't want this in the future.
Merge pull request #1239 from lavalamp/fixApi

Proposal for solving api plugability