main: retrieve config from env vars
etcdserver: use Action to fix possible data race
main: add test for setting flags
Merge pull request #1097 from unihorn/130

etcdserver: use Action to fix possible data race
Merge pull request #1040 from jonboulle/1040_config_env_vars

etcd needs to be configurable through env vars
etcdserver: use simple PRNG for GenID