MB-8069: Use json format to transfer vbucket map list

It is less error prone to use json library to marshal and unmarshal
vbucket list with target node. Currently vbucket-list support two format:
1. dictionary with recovery node map
{
  "node1": [30, 231, 445],
  "node2": [44, 500]
}

2. list with plain vbucket list: [30,32,34]

Change-Id: I7d74e4d0ba448dcb4f5cb80a829b43c78028d2ff