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
One would think this would work; but, I think that `path.Join` will remove a crucial slash here.

`path.Join("http://foo", "bar") == "http:/foo/bar"