The current registration of admission plug-ins happen too late so the
command line help is not yielding correct/useful information to users.
This is already causing confusion as mentioned in some issues.

This patch fixes it by moving plugins.go down to options package because
"plugins" in this context are themselves options for users. Registration
of plugins is not an expensive operation and it is already done in most
execution paths.

In future, we may want to revisit these plugins and migrate them to the
shared apiserver repo when appropriate.