Feature request: Support etcd DNS/SRV discovery

[jsok]

Rather than specifying a static list of endpoints in the etcd config, allow specify a SRV domain as per the DNS discovery protocol.

The etcd/client library supports and implements a Discoverer protocol which should make this relatively straight forward to implement.

Config change

Supporting a new config option which would be mutually exlusive with endpoints:

storage "etcd" {
  discovery_srv  = "example.com"
}

[raoofm]

Really need this.
we configure, etcd, etcdctl, proxy using SRV but can't configure vault via discovery.

[raoofm]

adding @xiang90