etcdserver: apply config change on cluster store

goal:
make changes on cluster store according to config change

non-goal:
check on adding exsiting node / removing non-existant node

for #1016 
@xiangli-cmu the name `create` indicates create-then-add the node into the list, so i think it describes the behavior better. And it follows the naming from store API, and CRUD(create, read, update, delete)(we will use update too later)