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 
i will add the check for ID.

i am still confused about the argument change. i pass `Member` struct into `Create` is to pair `Cluster` (multiple `Members`) that is returned from `Get`.

Current workflow is as this:
`AddMember` gets new `Member`, marshal it into json and let it go through consensus. When it comes back, unmarshal it into `Member` and pass to `ClusterStore`. `ClusterStore` marshals it into json and saves to the store.

some pseudo code is welcome