Convert nodepool status maps to lists

[wallrj]

• https://github.com/kubernetes/community/blob/930ce655/contributors/devel/api-conventions.md#lists-of-named-subobjects-preferred-over-maps

Discussed in #2004 and elsewhere. There are no maps of subobjects in any API objects. Instead, the convention is to use a list of subobjects containing name fields.

Change the API and add a conversion between the old map and the new list.

Update controls to deal with lists.

/kind bug

[wallrj]

Part of #311