refactor kube plugin's Action system

Cleans up gdt-kube's execution of kube actions to pull all assertion
evaluation up to the `Spec.Eval()` method.

Functionally, the only difference this patch introduces is that retries
are now performed in the `Spec.Eval()` method instead of only in the
`Spec.doGet/doList()` methods. What this means is that all kube client
calls, including Create, Delete and Apply, will be retried by default.

Signed-off-by: Jay Pipes <jaypipes@gmail.com>