add etcd2 backend, support etcd cluster and tls

[woshihaoren]

ETCD_CERT_FILE
ETCD_KEY_FILE
ETCD_CA_CERT_FILE
use this env to set tls

suport cluster, like
etcd2://etcd1:2379,etcd2:2379,etcd3:2379/xxx

[argosback]

Hey @woshihaoren — this PR was the direct inspiration for the new etcd2:// backend in https://github.com/fayrus/registrator. It uses the official etcd v3 client (go.etcd.io/etcd/client/v3), supports multiple endpoints via ETCD_ENDPOINTS and TLS via ETCD_CERT_FILE, ETCD_KEY_FILE, ETCD_CA_CERT_FILE — exactly as you proposed. The existing etcd:// backend is unchanged. Credit is in the changelog.