introduce cluster store

TODO

- [x] Rename Peer to Machine
- [x] Parse the new space separated bootstrap-config format
- [x] Ensure that the NodeName.ID()'s don't have collisions
- [ ] Generate initial cluster ID from a similar hash of the bootstrap-config
- [ ] Write documentation on static bootstrap

Considering machineKVPrefix is used here
https://github.com/coreos/etcd/pull/1147/files#diff-07c1b821da631838725545c3b13bc2c3R47,
it is weird to add indirection here.