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 want to split the add case into create and update:
create is used at the first time that the node joins the cluster
update is used for update peerurls and related stuffs
they are different in raft level because the first one needs to go through consensus and change the quorum, but the second one doesn't.

The split could make the logic clean, so etcd knows what it is doing exactly.