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
having seen this PR I fixed the import path to be github.com/coreos/discovery.etcd.io, sorry about that :-/