Add a description of pod states in kubernetes.

Closes #1146 
I am ok with the pod stays at "scheduled" here. Also with my pending PR, the container will stay with Waiting state associating with detail: pulling image .... 

Here is the question, how long we should tolerate this hanging or waiting at Container leve? and "scheduled" at pod level? Introducing a timeout value, or let high-level observe all state / status gathering by node, and make decision?