etcdserver: not get cluster info from self peer urls

Self peer urls have not started to serve at the time that it tries to
get cluster info, so it is useless to get cluster info from self peer
urls.
Merge pull request #1667 from yichengq/213

etcdserver: not get cluster info from self peer urls
integration: wait cluster to be stable before return launch

The wait ensures that cluster goes into the stable stage, which means that
leader has been elected and starts to heartbeat to followers.

This makes future client requests always handled in time, and there is no
need to retry sending requests.
Merge pull request #1668 from yichengq/214

integration: wait cluster to be stable before return launch
etcdserver: collapse shared readWAL logic
Merge pull request #1655 from jonboulle/wal_logic

etcdserver: collapse shared readWAL logic
wal: propagate errors
Merge pull request #1661 from jonboulle/wal_errors

wal: propagate errors
doc: add doc for proxy
integration: add basic discovery tests
Merge pull request #1670 from yichengq/207

integration: add basic discovery tests
etcdmain: actually return errors
Merge pull request #1680 from jonboulle/fix_errors

etcdmain: actually return errors
doc: add fallback to discovery section in clustering.md
etcdmain: do not exit inappropriately
Merge pull request #1681 from jonboulle/fix_exit

etcdmain: do not exit inappropriately
Merge pull request #1671 from xiang90/proxy_doc

doc: add doc for proxy
proxy: return JSON errors
proxy: add a todo for logging
Merge pull request #1685 from xiang90/proxy

proxy: return JSON errors