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 
@unihorn `CRUD` has nothing to do with naming the actual operation as far as I know. The store is a totally separate thing, and we do not need to worry about it. We use `Add`,`AddMember` all over the place in etcd. There is no difference between `Add` and `Create` in this case.