make discovery base URL configurable

According to #3

Add three env for init leader detection and server peer communication etc.

1. DISCOVERY_ROOT_URL : default https://discovery.etcd.io
2. DISCOVERY_ORIGIN_ADDR : default http://127.0.0.1:4001
3. DISCOVERY_INIT_LEADER : default 127.0.0.1:4001
It would be great to use this image instead: https://registry.hub.docker.com/u/google/golang/